Okay, how about this: If the chromatic character you want fic for is not listed in the options choose 'Any' and include an alternative character option in your request, saying how much you'd like fic focussed on that chromatic character in addition to the ones listed, and use a freeform tag like 'chromatic character'.

(Btw, I'll have to discuss with the rest of the mod team before implementing any changes though so this is just brainstorming possible changes.)