Wondfo OPK help please!!!

Do OPKs get darker as you approach ovulation, or is it faint one day and positive the next? Last cycle, mine gradually became darker and I had a positive by cd13. It's cd16, and I've only had faint OPKs. I'm so confused. I've had normal 28-31 day cycles, but this is only month 2 of OPKs. :(