[im a model]

to help you (or whomever youre proving this to) understand it better, think of it not as two red balls, but a red and a green ball. and the guy says, "one of the balls is red or green." then it becomes clear that the only thing that this information changes is the one instance where there is no red or green ball (the one case where there is a black ball and a blue ball). only when you think of the two red balls as being "the same" does the problem seem difficult.