A crochet castoff doesn't need extra wool but tends to be tight. I would either add one extra stripe in another colour and castoff at the end of that stripe OR undo 2 rows and use that extra bit of wool to cast off with (unless you work the crochet edge from live stitches, in which case the edge colour would take care of it).
If you are adding a crochet edge say in colour blue, casting off with blue should look okay. Whether you like the look of a crochet edge in a different colour is entirely up to you. If it all the way around, that will help make it look more integrated as well. Remember that crochet (apparently) uses a lot more wool than knitting does.