Are you using tank water, or a prepared 4dkh solution?
If tank water, stop the practice and get or make a reference solution. Tank water will give you unreliable results. The 4dkh in a drop checker must be exclusivly from carbonate, nothing else.
If you are already using a prepared solution and it was homemade (baking soda and distilled water) you may have had some moisture in the baking soda (it can hold a lot). You need to bake it in an oven (325 F for an hour or so) then weigh it out for the solution. I made this mistake and got basically the same type of results you described, a greenish blue instead of blue when I added the PH reagent. It will cause you to misread the CO2 in your tank, thinking it is high enough when it's really too low. My result ... algae bloom.
Pat