About adding a colored design to your sweater--if you do decide to knit in the round, doing stranded color work is actually easier than doing it flat. Knitting intarsia in the round is another story. It can be done, but from my understanding, it's not easy (I've never attempted it myself, so someone else out there could probably give you a better idea about that.)
If you did want to do a motif that would have required intarsia to do it's really easy to do duplicate stitch on stockinette in order to add the color afterword. This illustrates how to do that:
One thing about duplicate stitch to keep in mind though is that if you use it on large sections of your knitted fabric, then large sections are going to be twice as thick. So that's something you should keep in mind, especially if you are using a heavy and/or chunky yarn.