Considerations:
* many people (at least toddlers, people with dementia) are going to press blue roughly by accident. See the lizardman constant
* other people will not want to be responsible for any deaths and will press blue out of a sense of moral imperative
* many other people are going to take this into account and vote blue out of hopes we can save everyone
You should vote blue.
Puzzles like this are based on assumptions like all participants are rational adults with their full faculties.
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The first point is interesting. You could fork the question over this and have a few variants:
1.) The pure form where the button presses and restricted to legal agents (i.e. people with credible legal standing over their choices). 2.) The mixed form with the caveat listed here inclusive of all humans whether they are even physically capable of pushing a button. 3.) you could also go for a more expansive scenario that takes 2 to the extreme and includes animals as well.
1.) gets to the game theoretic form of the question. 2 muddies things, and 3 sets up a case for blue since the non agentic voters asymptote to 50-50 and a slim edge is morally preferable to killing half.