So here's a curious thing about that purple crochet hook.

The pattern for the project I'm working on recommends an H / 5 mm hook. The yarn I used recommends a knitting needle size of 5-6 or 3.75-4 mm. Frequently yarn balls give only knitting needle sizes (though this is changing among more enlightened yarn producers). To figure crochet hook size from a recommended knitting needle size, bump up the mm by at least 1.