Doom is the nickname of a clone trooper commander during the Clone Wars. He wore a unique set of Phase II clone trooper armor composed of a green, yellow, and gray color scheme. He was involved in the battle of Ringo Vinda.
I don't know how, I just remembered that in Clone Wars, in the Zygerian-arc when one of the Zygerians knew that Obi-wan wouldn't kill him, because Jedi don't kill unarmed enemies and Rex kills him and says: I am no Jedi. I AM NO JEDI... When will Star Wars stop ruining me? WHEN?