My cat just likes the bala shark, lol. Not sure why. If I move it to a different tank without her watching, I know she would go around searching til she found him, lol. Doesn't pay any attention to any of the other fish. It could be the color, shape, or size that get's their attention. As for the color, just talking about brightness and darkness, as I don't think they can distinguish colors other than light and dark. Or maybe shiny.