The standard roulette table is laid out as shown below (with some casinos including a '00' variant):
Take a look at the layout of the '12's' - each of the 1st, 2nd, and 3rd are equally divided between red and black numbers. Take a look now at the columns - the first (1, 4, 7, etc.) is equally divided too, but the second (2, 5, 8, etc) has 4 red and 8 black spaces, and the third (3, 6, 9 etc) has 4 black and 8 red.
To take advantage of this slight imbalance you need to place two bets per roll - red and the second column, or black and the third column.