One button means you almost certainly contributed to homicide, since the odds of everyone pressing red is essentially 0%.

The other one does not contribute to homicide.

The right answer, by the way, is to not press either button. "The only winning move is not to play."

Let me frame it another way and see if you still consider it homicide:

There's a cruise ship that needs to have a certain weight in order to not capsize. That weight threshold happens to be at 50% of the population (for whatever population we're considering in the original question). If the ship capsizes, everyone on it dies.

You're given the option: either get on the cruise ship or don't. Not to take an actual cruise, not for some other intrinsic prize, just file on it for a minute and then get off.

I don't see how those who refuse the risk of dying on the ship are complicit in the deaths of those who willingly choose to hop on it knowing the risks involved