Every color has 18 numbers. On roulette there is red and black. There is also a 0, which doesn’t belong to any color. If 0 comes, and you bet on either red or black, you won’t win. So, with 0, there are 37 numbers on the roulette.
There is also an American version, where there are 38 number while the another number is also null and is marked 00. Because of the second zero is this roulette unsuitable for the Martingale system, because it decreases the chance of winning.
So, basically you start betting 1€ on red. If you lose this bet, your next bet is 2€ (you have to double your bet) on red and so on until you win.
This betting system is very dangerous to play, beacause it requires the player to have a huge bankroll, and at the end you never gonna win, because there would be 10-12 reds or blacks in a row and your bankroll is gonna be busted..