There is usually a green 0 and a green double zero. If you play the inside there are 38 numbers including the two green ones. If you were to put a chip on the green number zero you would have a 1 in 38 chance of having it land on the green. I believe it pays out 36 dollars for every one dollar you bet on the outside. Because of the green zero and double zero, your odds aren't as good because that is one more color that the ball could land on if you are playing the outside (red or black)

The way you bet and the odds can get deeper than that but that is a quick explanation.