Skin color. You'd think it wouldn't be important, but it is. Don's the ugly one. He's spotty, his green fades to brown in places. Leo's too good looking, with uniform color all over in a nice light green. And yeah, I know, us liking uniform skin color comes from imprinting humans and their uniform skin color all over. Whatever.
Don used to have a theory. It was that we all mutated 'cause our human handlers in the pet store left DNA on us. I used to think I was handled by a black dude 'cause I'm a darker color than my bros. I said that to Don once and he smiled and made some wise ass crack about meeting the racial profile.
... Fucker. I returned that he must have been handled by some Asian kid 'cause he thought he was sooo much smarter than the rest of us. Leo overheard the comment and suddenly we were in a three way argument. I called Leo a privileged white boy 'cause Splinter always favors him over the rest of us. The entire thing ended up degenerating into pointless racism which doesn't matter anyway because we're fucking turtles.
You know... I wish I were black. Then I'd be in a minority of several million and not just four.