Colour-blinded

Confidently, you reach out to count 129 tokens and separate them from the others, and then turn them over.

That simple? Yes. You see, the number of green tokens in the group of 129 tokens must be the same as the number of red tokens still left in the original group. It's obvious when you think about it; if you got all of the red ones in your group, there are none left in the first group, and once you turn yours over, there aren't any in your group either. If you didn't get a single red they're all green, and when you flip them over you get 129 red tokens, just like in the other group. And the same goes for any number of red tokens you remove.