If you’ve purchased the API Freshwater Master Test Kit, you have probably noticed that the colors for 10 ppm and 20 ppm on the nitrate test are pretty much the same. I think it must have been a misprint on the multi-test color card.. When my nitrate test solutions ran out, I purchased the individual nitrate test kit. It includes it’s own color card and the 10⁄20 difference is quite distinct. Then 10 ppm color is more yellow, the 20 ppm color much more red. The other colors are also different, with 5 ppm being more yellow and 160 ppm being very brown.
I couldn’t get a great shot of the two cards, but hopefully this is good enough to keep you from pulling out your hair. If you have hair.