Plastics will filter light, so red button + blue light (most likely) != purple, but going clear with RGB LEDs would work and wouldn't look like a half-assed compromise. Hmm.
IIRC, TMNT cabs used a pair of Coin Controls mini-over/unders with the standard red rejects, although I have seen people's restorations with a single 4-slot Happ door with coloured rejects. Too bad, because Exidy used those Coin Control doors (so the reject size would be the same), but I haven't seen a TMNT with those door AND coloured rejects.
I am not willing to futz with the door beyond changing out the inserts and changing the bulbs to LEDs.
I am toying with the going green if I can find green reject buttons, but if I can find clear reject buttons, I think your RGB LED suggestion will be the winner.
Thanks!