It's not 50 red and 50 green. There are 100 total balls, but you don't know how many are red or green. The number of reds is drawn from a uniform distribution,

It's not 50 red and 50 green. There are 100 total balls, but you don't know how many are red or green. The number of reds is drawn from a uniform distribution, meaning the urn is equally likely to contain 1, 10, 55, or 99 red balls.