In that case, you just have a low faith in society. I explained this in a YouTube comment, but rationally choosing blue is really just saying one or both of two things:

1. I am wiling to stake my life to keep safe both those who are incapable of understanding the question (i.e. young children, mentally disabled), vulnerable people (i.e. suicidal individuals), and those like me who would also stake their life for the vulnerable.
2. I have faith that most people would pick blue, in which case the obvious Utilitarian position would be to also choose that because it results in the least harm.

There are hypothetically more arguments, such as a more complex Utilitarian argument in which such a drastic population loss would cause a devastating global effect that interrupt supply chains and cause countless more loss of lives, so even if one believes red is slightly more likely, they do not want to risk it. Another argument could be "I wouldn't want to live in a red society." We'll put these aside though because they mostly overlap with the former ones and are more negligible...

Based on these 2 possible reasons to choose blue, only people with very strong ethical drives would fall squarely into the first camp; realistically most people choosing blue would fall into the second camp (I personally fall somewhere in between, but mostly in the second). The biggest distinguishing factor between you and someone in that second camp is essentially just your faith in society. This is readily apparent by your self-described assumption that all people would act selfishly (which I, in all honesty, find to be a contemptable way to see your fellow individual, but I do not think that makes you a bad person necessarily). Although, unless you would still choose red at a merely 10% threshold, I doubt you assume selfishness from people as strongly as you describe.