The probablilty of a two tone flop is 0.5506

The first card dealt can be any card, so the chance of dealing this is 52/52.

For a two tone flop the second card has to match the suit of the first. 12 cards do this, so the chance is 12/51.

The third card can't match the suit of the first two or there would be a monotone flop. There are 39 cards that do not match the suit of the first two, so the chance is 39/50.

So the probablility of a two tone flop is 52/52 * 12/51 * 39/50 EXCEPT that this just calculates the probability of a flop where the first two cards have the same suit. (ie SSX) We also need to include SXS and XSS flops too.

So the probablility of a two tone flop is 52/52 * 12/51 * 39/50 *3 = 0.5506 = 55.1%