I have always wondered about the effect of the reagent on the readings in a drop checker. To check this I did this simple test: the test tube for a pH test kit holds 2 ml of water. With 4 dKH distilled water in the tube, I added one drop of reagent - blue. 2 drops - blue. 3,4,5 etc. drops - all still blue. This convinced me that the reagent is neutral as far as any effect on pH is concerned. I did some reading, and found a DIY method for making pH reagent from bromothymol blue, a dye. In that method, the last step was to add a significant amount of a alkaline substance - I forget which one, but perhaps it was sodium hydroxide. I presumed that this is what makes the reagent neutral.
Another test I did was with a gas permeable membrane drop checker, where there was only a few drops of 4 dKH water in it, plus a drop of reagent. In my aquarium, that DC agreed with my blown glass model DC. That reinforced my conclusion that the reagent doesn't adversely affect the results. But, it is clear that if you had one drop of 4 dKH water, and 4 drops of reagent, for example, it would likely not be very accurate. That just seems logical.
This morning I removed my DC from the tank and stuck it on the outside of the tank, to see how long it takes for the color to go back to green. So far, after 2 hours, it is still yellow.