Avalon's guide to API test kits (for fish):
Ammonia & Nitrite: If it changes color, you have too much.
Nitrate: If it is yellow or light orange, you're good. If it's dark orange or reddish-orange (and any further degree of red), you have too much.
GH: Throw it in the trash.
KH: If it changes color, the drop it changed color on is the degree of KH.
PH: Dark blue is around 7.8-8.0+, medium to light blue is 7.2-7.8, light greenish is about 6.8-7.2, yellow is battery acid
(I kid, but it's pretty low)