I'm generally wary of proposed overhaul of systems already in place but, yes, i think color changes should be doable. Adding a menu to Fouvia would be simplest. Even two-tone shouldn't be a great obstacle.

I know puppets have their gear changes, but i suspect even simply bardng the wyvern (i like that word even though they aren't mounts) would run into problems with the ps2. and at the risk of another 'stay OT' as long as we're wishing why not go whole-hog and ask for an adventuring fellow style of wyvern mod. choose the character model, name, color, and personality.