I'm currently looking at 255 rainbow ponies using more embark dwarves, and when I'm done I'm going to check the legends for their civ, see what factors they might have in common. Almost all of them are very consistent in their colouring, but some castes have a fair bit of variety, or at least one thing will vary while everything else doesn't. I'm also finding some prefstring and pluralisation errors, so I'll fix those up. Another problem with cutie marks is that they'd have to be a colour, which unfortunately means we'd have ponies who "like the colour a wheel of cheese" and demons and night creatures that have cutie marks as their colours, like how I'm already seeing ponies who like the colour starry or eclipse black, and get eclipse black demons showing up in preferences. Unless there's a tag for not using a colour unless it's called by name, this could be a problem.
EDIT: Here's all the data from those ponies:
Earth F:
Coat: moss greenx15
Hair: teal-pearlx15
Earth M:
Coat: crimson, charcoal, burnt umber, midnight blue, dark chestnut, mauve, black, green, gold, brownx2, cerulean, ochre, chartreuse
Hair: dark indigo, lavender blush, fern green, charcoal, dark peach, crimson, teal, gray, maroon, dark olive, green, dark chestnut, blue-dark indigo-fuchsia, ivory
Peg F:
Coat: sky bluex13
Hair: redx13
Wings: emeraldx13
Peg M:
Coat: moss greenx10
Hair: rustx10
Wings: light bluex10
Uni F:
Coat: dark scarletx11
Hair: dark peachx11
Uni M:
Coat: sandy taupex18
Hair: lilacx18
Royal F:
Coat: pumpkin
Hair: brown
Wings: fern green
Royal M:
Coat: amethystx2
Hair: lavender blushx2
Wings: copperx2
Keshi F:
Coat: emeraldx14
Hair: pine greenx14
Keshi M:
Coat: scarletx11
Hair: indigox11
Vamp F:
Coat: maroonx12
Hair: greenx2, cardinal, aquamarine-amber, indigo-maroon-chartreuse-lemon, jade, whitex2, indigo, dark violet, maroon(Yes!), gray
Wings: maroonx12
Vamp M:
Coat: dark peachx12
Hair: fuchsiax12
Wings: black, olive, scarlet, orange, cobalt, green, redx2, ivory, chartreuse, pine greenx2
Tent F:
Coat: goldx14
Hair: plum-red-emerald-yellowx14
Tent M:
Coat: dark peachx16
Hair: cobalt-dark violet-chestnutx16
Dark F:
Coat: amethyst, green
Hair: blue-amethyst-fuchsia, dark taupe
Wings: maroonx2
Dark M:
Coat: sea greenx2
Hair: pine greenx2
Wings: ivory, vermilion
Draltha F:
Coat: olivex11
Hair: green-yellowx11
Draltha M:
Coat: auburnx18
Hair: yellow, copper, tan, green-yellow, dark scarlet-dark-pink, brass, carmine-azure-cardinal, midnight blue-pearl, burnt umber, bronze, mauve taupe, teal-white, amethyst, indigo, cerulean, pale pink, black, burnt sienna
Slippy F:
Coat: beigex12
Hair: cream-saffronx12
Slippy M:
Coat: azurex13
Hair: charcoal-pearlx13
Gorlak F:
Coat: pucex11
Hair: slate gray-dark violetx11
Gorlak M:
Coat: pale brownx15
Hair: spring greenx15
Underlord F:
Coat: ceruleanx3
Hair: light brown, dark tan, gray
Underlord M:
Coat: pinkx3
Hair: carmine-crimsonx3
The world is five years old, I'd genned it for testing something. Their civ, The Long Beard, started out with a female tentacle pony, a female keshi pony, two male pegasi, a male gorlak, a male darkbolt, a male draltha, a male keshi, a female royal pony(i.e. regular royal), and a female slippy pony. No additional members appeared to show up during the five years of world gen.
The only caste that varied across all descriptors was male earth, which didn't show up in world gen and is the [MALE] caste, so we may have a similar issue to the trap bug in Dwarf Therapist, where only male earth ponies are marked as male. The castes that had some variation were male and female darkbolts, male dralthas, both genders of dark royalty and female underlords, but only two of these showed up in world gen.
Basically, what I think is happening is the [MALE] caste, male earth ponies, is marked as Caste 0, and every other caste is given a number. When variation is checked, it checks Caste 0 for variety, then Caste != 0, and it sees that the second set has twenty three varieties after one pass so doesn't add any more full colour schemes to the castes included in it, though it mixes up the hair or wing colours occasionally for a little variety.
I don't think bypassing it is going to work, I think descriptors are hard-wired to be genetic. What'd be great is a tag to make a particular descriptor non-genetic.
Highlights of the descriptions where a female darkbolt who was totally maroon, and the male Underlords being utterly fabulous.