Sounds like something is not right. Are you injecting CO2? What is the pH/kH of your source water (presumably your tap)?
If you put your aquarium water in the drop checker and then add the pH drops (the same stuff you use with a pH test kit) - you are just doing a continuous pH test. The color will have no bearing on your CO2 - at least not with the standard blue/green/yellow legend. You will have to test for your kH and then determine your pH from the color in the drop checker. Also, if there are any other elements in your water that mess with your pH/kH/CO2 relationship - this method will be effectively useless for measuring your CO2...
By using a kH reference solution calibrated to 4dkH instead of tank water - then your color legend will follow your CO2 levels. It is made to maximize the readability of the colors given by the pH test drops - setting solid green to 30ppm (ish). I keep mine light green to be on the safe side.
You can buy a premixed reference solution - probably off aquabid - which is highly recommended.