I primarily make amigurumi, so that means a lot of work in spiral rounds. I constantly find it irritating when I want to do stripes because you always either (a) have to switch to joined rounds or (b) put up with the jagged colour switch when you go from one colour to the next. However, I recently figured out a way to get a two-colour stripe in spiral rounds, with no joins and none of those ugly jagged edges.
The picture above shows a Nintendo DS Lite cozy I made recently with this technique; you can't really see all around it, but you can see where I started the white coloured stripe, and how there are no joins above it!
The way I did it was:
1. Complete one round in one colour, put a stitch marker in the loop left on your hook.
2. Remove the crochet hook, join second colour in the first stitch of the last round, then begin a round in the next stitch, counting the join as the first stitch of the round (e.g. if you're doing all sc, join colour 2 into first stitch with a sl st, then sc in the next stitch).
3. When you get to the last stitch of colour 1, put a stitch marker in the loop of colour 2 that's left on your hook.
4. Remove the crochet hook and insert it back into the loop of colour 1 and carry on with the round.
Repeat steps 3 and 4 for as long as you need. When you get to where you want to finish one of the stripe colours (in my picture, I "finished" the white and carried on with the blue), in step 3, you just make the last stitch a sl st instead of a normal sc, finish off and weave in the end. Then when you come to that stitch on the next round, it's a smooth transition back to the beginning of the round.
Presumably, this technique can be extended to as many colours as you like, although I've not tried it myself. Happy hooking!