The feather colours thus far have been red, orange, yellow, green and blue. (You're right, no-one has gotten beyond blue yet.)
These colours are following the visible light of the electromagnetic spectrum in order from longest and slowest wavelength to shortest and fastest. If this trend continues the next colour will be Indigo Feather and then Violet Feather. These levels will be reached by 6,000 and 7,000 posts respectivley. According to Myriads he has levels for up to and including 50,000 posts. What he'll go into after he runs out of visible colours of the rainbow is anyone's guess. Given the guy's brain though, I expect it'll be something like ultra-violet etc.