I some times have trouble with the color comparing too. I use the aquarium Pharm. liquid tests, and it does help if you stand right up next to a window, or go out side. I don't test my water as much as I probably should. When I do, I basically test to see that my Ammonia is 0 (pretty easy to tell that one) nitrites are 0 (again pretty easy to tell) and that my nitrates are low. If you are testing your water regularly, you will notice if there is a big swing in your ph
or ammo,etc, and I think that is really the main purpose of these tests.