When you start with a crochet cast on like this you can also knit with another color after you've cast on. The original cast on color becomes a provisional cast on and it's easy to take out.
Provisional cast on is used when you want to insert the needles back in your knitting at the beginning of the work to continue in the other direction or bind off so you have a matching bind off on both ends. I've used it many times for hems on hats.
Here's a picture. The hat on the upper right is hemmed.