Clomid, ovulation

I'm taking clomid and I don't really think I've ever ovulated on it, well I have never gotten positive opks. Well do opks darken as you get closer to ovulation? I took one yesterday and it was so faint, this morning I took another one it was still faint but darker than yesterday and the day before.. I'm on cd 13, I'm really hoping this works, and I think this cycle I am ovulating, it's exciting, but nerve wrecking, I'm scared it won't work.. Any one know if opks darken the closer you get?! Every time I've taken opks they're the same boldness every test, except this time around.. Thanks!