Traditionally, we think of stranded knitting as being done in stockinette and in the round, usually for making garments like hats and sweaters. The significance of this is that typically you only ever need to use the knit, because stockinette is achieved in the round by knitting every row.
So what happens if you want to knit stranded colorwork flat? The idea may be alarming at first, because to knit stockinette flat we must alternate rows of knits with relatively pesky purls. What does stranded purling look like?
Good news: there’s no reason to fear! Remember that when we purl, the yarn is held to the front of the work — which in a flat piece is the [wrong side]. As long as we make sure to hold all strands to the front of the work, you can continue carrying your unused yarn along the wrong side and forming floats as usual! The only “trick” here is that you’ll have to get used to purling with two strands, just like you had to practice knitting with two strands at first.