Between 5 & 10, Easiest way is to hold the Vile up to a blank piece of printer paper. Also keep in mind the Nitrate test works a bit differently with API Shake bottle 1 for 30Sec add 10 drops, cap the vile invert for 15-20 sec, add the next 10 drops after shaking 30 sec and invert the vile for 60 secs then let and sit for 5 minutes.
No it's been up for a while but every time I test I can't tell between the colors!!!! Why do they make the colors the same hue? Makes no sense!!! No nitrate issue. Hehe I just love that my tank is about 5 ppm not used to that. Learned I can't and shouldn't slack on my water changes.
Your confusion is why I don't like API's nitrate test. A lot of folks recommend Sailfert, but IMHO it's not better. The three most reliable and easy to read nitrate tests are: NYOS nitrate test kit (which uses a second vial of clear water to help you compare colors) Red Sea's Nitrate Test kit (More work but much easier to read) and a nitrate probe. The probes are nice, but require constant calibration to keep accurate.
Take the vial, and look through it, thats the easiest way to discern what color it is. Then again, when your that close, its not a bad thing either way.
Do yourself a favor and throw those test kits in the garbage. Not trying to be rude but they are the worst on the market. Get yourself some good quality test kits and you will be happier in the long run. [emoji106]
There are many different tests, there not to bad priced in my opinion, the nitrate and phosphate tests are really easy to use. Sailfert is good too though