The overnight yellow color just means your tank doesn't outgas very fast. You may not have much surface disturbance, for example. I always tried for a lot of surface ripple when I used CO2, so it would outgas more rapidly at night, and so the water would absorb more O2. I don't see any mistake you are making with the drop checker. It is a very simple device, a simple method to use, and foolproof, as long as you can discriminate colors reasonably accurately. But, it will never tell you any more than that the ppm of CO2 is somewhere between about 20 and 45 ppm, and even that is only where the drop checker is located, not other places in the tank.