They will eat or drink out of the other colored cups. If i have them up higher than ground level they will pull them down, probably competition...except Bleu will bathe in the green cup no matter where it is. Peri seems like he doesn't know how to take a bath. He will just kind of sit in the water but not do anything LOL so i think his game is a way that he can not be so jealous of all the fun Bleu is having.
Hubby has a theory like yours. He thinks the flourescence of the color attracts them to it.