The test is designed to be read at a certain time limit (or if using a titration test, a certain drop limit). If you leave it sit, it will turn darker/lighter and will also turn cloudy.
Dont read anything into it, when the timer went off last night-that was the correct reading. (The longer the solution sits after the timer goes off, the further away from "accurate" youre going to be)
Personally though, if you want my opinion, dont use api kits. Ive never used them because Ive heard so many negative things about them. I like salifert. Theyre more expensive but for me-more accuracy is worth it. Especially the amount of money we put into these tanks.