This can't be a new idea, but I did not see it when I searched. I painted the backside of my Drop Checker with appliance/tub touch-up paint to improve the contrast. This stuff is nasty (s/b used outside) and dries very quickly. I got a clean line by taping and removing the tape while still tacky.
I'll attach a pic, solution is still blue, but will end up almost clear (yellow/green).
I'll attach a pic, solution is still blue, but will end up almost clear (yellow/green).