Look at it this way. Deal 2 cards off the top of the deck. The first one will be one of four suits. There are now 13 of each of 3 non-matching suits, and 12 cards that match. That is very nearly 1 in 4 cards that will match. So, first time I get a diamond and a spade, and I lose $1. Second time I get a diamond and a heart and I lose $1. Third time I get a diamond and a club and I lose $1. Fourth time I get a diamond and a diamond and I win $3. I'm now even. So the casino advantage on that play is the difference between 12 cards and 13. The odds are 3.25:1 and they pay 3:1. About as close to even as any bet in the casino, even not counting the royal.

As for the royal, the odds of getting the first half of a royal is 13:2. The second half is 51:1, making the odds 331.5:1.