Now that you're comfortable with double knitting, it's time to get to the fun stuff, which is making a two-sided, mirror-image color pattern. This is the whole reason we do double knitting, right?
If you're following along with my swatch, which has 12 stitches on each side, I worked three stitches in the original color (that's six stitches total, three on each side), then six stitches on each side in the opposite color, then three stitches on each side in the original color again.
To change colors you simply knit or purl with the opposite color than the one you have been using. In the picture I'm on the yellow side and I want to start working my gray square, so instead of knitting the stitch with yellow, I knit it with gray. The next stitch I will purl with yellow to start what will be a yellow square on the other side and so on across the color section.
To finish with yellow at the end of the row, just start knitting the stitches on that side in yellow again and purling in gray as before.