"Indigo and Green" explained

I've seen all kinds of wacky theories on why they called each other this, like they were influenced by Manson giving the girls colors as names. It's really pretty simple - in multiplayer Doom, each player has a color to differentiate them, and two of those colors are indigo and green.

Doom Wiki - "Player" - "In a multiplayer game, the other players appear as marines in differently colored uniforms and armor. In the Doom games the colors are green, indigo (gray), brown, and red"