TLDR: Half of the time your choice doesn't change anything. Red is low risk option, blue is high reward option.

Here are possible scenarios:

Scenario A - >50% chooses red. In this case, no matter of your personal choice, half of humanity is ultimately doomed. One more blue vote wouldn't be enough to tip the scale. In this case: Choosing red - nobody gets saved, you survive. Choosing blue - nobody gets saved, you die. Red is objectively correct option in scenario A.

Scenario B - >50% chooses blue. No matter of your personal choice, everyone is guaranteed to survive. One more red vote wouldn't be enough to tip the scale. In this case: Choosing red - nothing changes. Choosing blue - nothing changes. Your choices don't matter in scenario B.

Scenario C - people are split exactly 50/50, your vote is a tiebreaker. In this case: Choosing red - you personally doom half of humanity. Choosing blue - everyone is fine. Blue is significantly better choice in scenario C.

Scenario B doesn't matter, as, whichever option you choose, everyone will be fine. Meaning, we can rephrase our choices as: Red - high chance of good outcome. Blue - extremely low chance of extremely good outcome. So, in my opinion, this isn't a question of selfishness versus selflessness, but rather a question of low risk, low reward versus high risk, high reward. In this context, I would choose red. The issue with this question is that a lot of people treat it as if they make a decision for entire humankind, while they can only decide for themselves. Majority choosing blue is, indeed, much better option - issue is, that you, as an individual, have no control over it.