One extra consideration I haven't seen flagged: if you pick Red and the blue pressers die then everyone who's left alive definitely pressed Red. Then there will be a society-wide tacit agreement that this was a fine thing to do, blue pressers were all dumb and had it coming, and no-one needs to feel bad about it. So you won't even need to feel guilty.

First they came for the blue button pressers, and I did not speak out.

Red button pressing is killing people that didn't think/act like them. You don't need to feel guilty about it, because this type of thing happens all the time.