There are a ton of threads on these forums about using a drop checker...
The instructions that come with them are almost always wrong and tell you to put tank water in the drop checker plus the reagent they send with the drop checker.
You don't want to use tank water or the color the drop checker turns is pretty much meaningless.
Buy or make some 4 dKH water using distilled water and baking soda. Fill the drop checker about half way or a little higher with 4 dKH water and then add a couple of drops of bromothymol blue which is standard pH reagent, the reagent you get with the drop checker should be okay to use. Install drop checker in the tank. It will take about two hours to fully register a change. When using 4 dKH water the drop checker will turn green when you have approximately 30 ppm of CO2 in the water. This is a good level for the plants and a safe level for the fish.
Green means 30 ppm only if you are using 4 dKH water, if you use tank water green may or may not mean 30 ppm depending on what the kH of your tank water is and what is affecting the carbonate hardness. The 4 dKH water gives you a known standard to work with.
Most important thing to remember about CO2 is to never make increases if you are not going to be home to watch the tank for several hours. Increases in CO2 levels happen pretty quickly but a drop checker is slow to respond so you need to watch to make sure the fish are okay. If they start to lose color, look stressed or cluster at the top gulping air turn the CO2 back down and/or create more surface agitation so you get more O2 in the water for the fish. If the fish look really bad you can also do a quick 50% or so water change to reduce the CO2 levels.