Scientifically, morphology is the term for physical variation within a species. And it is true that with any animal, they don't all look the same. Which is why it's a plot hole that the only true variation within a species is the lucky slime. This game mechanic addresses that very plot hole, and makes your world interesting, by adding differences within species.
A Note About Largos
The various traits of a slime are contained to that slime -they will not inherit the other slime's traits, nor will largos formed from their plorts inherit their traits. As for sliding scale variation, that is the only exception.
Glass Frog Syndrome
This is equivalent to albinism in vertebrates -it's the effect of there being a lack of pigment. These slimes are clear, named because their translucency resembles that of a real-life glass frog. These slimes don't usually last long, because they cannot see anything during the day because the sunlight hurts their eyes.
While most quantum slimes have white eyes, very rarely, another slime may get walleye instead, even hunter slimes have yellowish pupils. Almost all of these slimes live in the ruins.
Permanent Hostility Syndrome
When a slime gets angry, their eyes turn red (hunter slimes instead get shrunken pupils) and their face turns dark because of a special type of blushing. These processes happen because of chemical changes. However, some slimes always have the chemicals that produce these responses, even when they are not angry. The result is that other slimes mistake that slime for being hostile and react accordingly.
Normal Size Variation
This is exactly what it says: slimes can range from smaller than average to larger than average. Smaller slimes tend to be more mobile and withstand heat better, while bigger slimes can endure damage more easily and withstand cold better.
This is when a slime becomes oversized. It is the size of a largo, and when it is a largo, it becomes even bigger. In addition, it's mobility is hindered due to all of that extra weight, making it vulnerable to predation. However, it is best known for the tendency of these slimes to eat until they become immobile, then, when that's over, behave normally but without eating until they become unhappily hungry, then they eat to immobility again.
Double Plort Syndrome
When a largo doesn't produce two/four seperate plorts, they produce one/two combined plorts, and any slime who eats these mixed plorts becomes a Tarr.
No Largo Weight Gain
Many phosphor largos have trouble flying. However, some phosphor slimes don't get that problem, and have the same mobility in the air as when they were small. Even better, a slime scientist discovered that it affects all slime species, and makes them able to bounce as high as they did when they were small. Cosmetically, it also causes phosphor slimes to retain their old translucency.
This is a rare trait that can occur among a pink slime, where the slime is colored like a puddle slime but duller, and males are paler. Largos will also be blue. It is believed that this is a leftover prehistoric trait that started showing up when puddle slimes were starting to become a species.
Leucism or "White Tabby"
The reason lucky slimes are white is because ingesting metal causes pigment loss. However, some tabbies are naturally born this way without trace metal, and aren't lucky slimes. This is a common mutation.
Pattern Contrast Variation
Males and females have different levels of contrast, but there is still variation. Some males have no pattern at all, while some females have almost black stripes. The general rule is that patterns are more distinct in the moss blanket and ruins and less distinct in the dry reef and glass desert.
Some tabbies are brown-red. This is believed to be a leftover trait from when hunter slimes started to exist, and is common in the moss blanket because of camouflage benefits.
Some tabbies have yellow eyes, also a leftover trait from the evolution of hunter slimes.
This is not a full-on hunter band. It's when there are additional stripes that form a band, but the band is not solid.
This is a rare variation that is thought to be one of the earliest evolutionary traits in the history of the species. It proves that not only did pink and tabby slimes have a common ancestor, but also that pink and saber slimes had a common ancestor. This trait is more commonly seen with the saber largos in The Wilds.
This is a common color variation.
This is a somewhat common mutation, akin to melanism.
This is when a tabby has small spikes -but not to the extent that a hunter has them. Strangely, there are the same gender differences that hunter slimes have -females with this trait have extra spikes on the sides of their body. These individuals appear most where hunter slimes are also present.
Varying Tail Lengths
Some tabbies have longer tails than others, while some tabbies have short tails -or no tail at all. In the dry reef, tail length is medium-length -because it needs to be suited for the cold winters but also for escaping. In the moss blanket, tails are long because they need to propel themselves away from various creatures and Tarr-infected slimes, and actual Tarr. In the ruins, tail length doesn't matter. In the glass desert, tails are short or absent because they can easily catch fire.
This is a variant of the tail length variation where there is no tail at all. Slimes without tails can't be grabbed by it and have one less place to catch fire. However, these slimes are less bouncy because they cannot use their (absent) tail to propel themselves into the air.
Varying Ear Sizes
In the glass desert, tabby slimes have large ears that make them look like fennec foxes.
This is when a tabby has a coiled tail that it uses to propel itself into the air, making it jump super high.
This is a problem excusive to tabby and hunter slimes. Individuals are completely missing their eyes, so they always look like their eyes are closed (the canon appearance lucky slime has this trait). They can only detect things by sound and smell, thus, they are at a greater risk for Tarr, other predators, and natural disasters.
Largo Mouth Change
Some tabbies have a genetic instability where their mouth changes when becoming a largo. Since tabbies have mouths that are adapted to biting prey, this makes hunting harder. This problem happens a lot more frequently in the ruins and glass desert, possibly due to malnutrition from either the sheer amount of slimes or the small amount of food.
This is a fairly rare leftover from when hunter slimes were evolving from tabby slimes.
This is when a hunter slime retains it's solid green color from when it is a kitten. It is a rare mutation that can appear anywhere. The coloring carries over to when this slime becomes a largo, not taking on the coloring or pattern of the other slime at all.
Pattern Contrast Variation
Unlike in tabby slimes, this does not depend on gender. Some slimes have fainter markings than others, and may even lack markings. This is a common trait in the glass desert, for blending in with sand.
While tabby slimes turn blue, these slimes turn black -when the coloring of their pattern carries over to their entire body. This variation is quite common.
This is when there is a band that only covers the area in between the eyes and a little bit after, the same as when the slime is feral, due to oddities with pigment. Unlike with Permanent Hostility Syndrome, other slimes do not mistake the slime as being hostile, because there is a distinct difference between hunter markings and "dark blush".
This is when a hunter completely lacks spikes. This becomes a problem when identifying gender, because normally, females have extra spikes on the sides. This variation is rare, because the other slimes can't tell either, and so these individuals don't reproduce. Curiously, when these slimes become largos, other gender-related traits are also missing, with the exception of slimes that rely on color/pattern.
This is when the tail is coiled like a spring, causing slimes to jump higher when they use their tail to propel themselves.
Tail Length Variation
You can tell where a hunter is from by the tail length. Hunters from the moss blanket tend to have normal size tails, while hunters from the glass desert have short or absent tails -because in the glass desert, longer tails can catch fire more easily.
A variation on the varying tail lengths.
When a slime is missing their eyes, so it always looks like they are closed. While this problem is not as common in hunter slimes as it is in tabbies, it can still happen. These slimes don't go invisible.
Rock Color Variation
Rock slimes are blue because the vegetables they eat contain minerals that can affect body coloring, especially indigonium. Some rock slimes are more sensitive to this than others, resulting in their rocks being colored in varying shades of purple.
In the ruins, most rock slimes have yellow rocks instead of the usual gray or purple coloring, caused by sensitivity to a different mineral. In addition, the slime will have a greenish tint to them. This trait is almost never seen outside the ruins.
In the glass desert, rock slimes are turquoise and their rocks are shiny and dark teal. This trait does not naturally exist outside the glass desert, or at least, it's extremely rare.
These slimes cannot see as far away as most other slimes, so they are always squinting. This problem only affects rock and crystal slimes.
Some rock and crystal slimes can't retract most of their body spikes, except their bottom and their face, making them look like a pinecone. This trait is somewhat hindering.
Variation In Rock Shape
Some rock slimes have wide rocks shaped like canon, while others have the narrow rocks shaped like a crystal slime's crystals, and then there is a range in between.
Where a lot of rocky spikes should be, there are a bunch of rocky scales, kind of like a mosaic slime, that form a smooth "helmet". This trait is not helpful, because the slime does not have enough friction when rolling, and thus starts going out of control. In addition, they have to fight like a feral slime, instead of using their (absent) rocks to attack.
Crystal slimes are not exempt from indigonium sensitivity, either. In fact, their body is even more sensitive, which is why they're purple, not blue. Since their body absorbs most of the indigonium, there is not enough indigonium left to color the crystals. Rarely, a crystal slime may not absorb the indigonium into their body normally, and so they are a bluish-purple color while their crystals are purple.
Like rock slimes, crystal slimes can also become nearsighted -they can't see as far.
Some crystal slimes cannot retract most of the body spikes that only appear when creating crystals. These slimes end up looking like pinecones with a face. This is an unhelpful trait because it hampers mobility.
No Crystal Production
This mutation is not obvious at first, but it can be dangerous. This is because on a hot day in a crystal slime's home environment, or when a crystal slime gets sick, then they have to produce crystals to cool off or else their body temperature will rise. When a crystal slime can't produce crystals, their body temperature can rise to dangerous levels. In addition, this slime will become uncomfortable when the temperature gets above 95 degrees, which can very easily happen in their home environment.
This is when a phosphor slime turns very, very dark blue. You can still see the light, however. This is a rarer trait, but not super rare.
This is when a phosphor slime is a purplish-pink color, evidence of evolution from the pink slime. This is a very common trait.
No Largo Wing Coloring
Not all phosphor largos get colored wings when they become largos -some have wings that remain white (or yellow).
There is a strange pigment variation where the pigment goes to the wings instead of the body, resulting in a white slime with blue wings. This carries over to largos. This trait can be combined with No Largo Wing Coloring to create a largo with the same color scheme as the small slime.
(Credit to Jojoto Pep Lol Yech for the artwork that inspired this trait.)
A purely cosmetic gene that makes a slime have pale yellow wings. It is recessive, but it's also incredibly common. This trait carries over to Reverse Coloring.
When a phosphor slime's wings are undeveloped, and appear as tiny stubs instead. Phosphor slimes don't fly because of their wings, they fly because of the fact that they can float -the wings simply enable them to have a much easier time moving in midair. Phosphor slimes without wings are much slower and low-flying than phosphors with wings, because they have nothing to propel them through the air. And no, tabby and hunter largos without wings cannot use their ears/tails to propel themselves.
This causes a bunch of problems, similar to when a hunter slime has no spikes: you cannot tell the difference between a male and a female, and the slimes cannot either.
This mutation causes phosphor slimes to have a second pair of wings, similar to a dragonfly's two pairs of wings. This causes phosphor slimes to be able to fly very fast.
Phosphor slimes fly because they can float, not because they have wings -the wings are there to move them through the air. When a phosphor slime is too heavy, then they can't fly. This causes them to be clumsy, as phosphor slimes are not adapted for ground movement. Note that this does not count how phosphor largos have trouble flying -those slimes can still fly and still rely on it.
When some of the inner bioluminescence goes to the eyes, creating a slime with glowing yellow eyes that don't pulse with the rest of their body. This trait is almost completely cosmetic, and it's quite common.