the only painting restrictions i have ever heard is basically having three colors on a model... besides that, you can paint your models any color and use and applicable rules.
if you must... just say you use a varient craftworld..w ith teh same rules, but differnet color. maybe you are jsut an iceworld detahcment. maybe you are a special trained unit... for example a jetbike unit in ulthwe colors could be jsut a small jetbike ulthwe force that uses the siam-hamm rules...
same w/ the catachen rules for IG... and all the SMurf chapters.
for exmaple... i USUALY play ulthwe. my seers wear blue red and yelow, with black highlights, my tanks and guns are yellow and red... and my guardains are a variant of the iyanden color... and it's totaly legal
hmm... i can think of one example though... "red paint job" for the orcs