Thinkers, hopefuls and desparates have been doing their heads in for years over that. I gave up on it, around 1967.
Here's another enigmatic challenge to consider, there are ten red~odd numbers and eight black~odd numbers, so if you wager on the R~O combination, you should have two extra chances on that spin.
This is all for the French (single zero) layout game. Interesting? Ringmaster.