This may be of interest. https://www.kickstarter.com/projects/iorodeo-colorimeter/educational-colorimeter-kit
Usually you don’t track nitrate directly but by a colour making reaction (oxidation), you then compare the optical density of your test solution with a series of known standards of known concentration and then " look up on a graph" your OD and hence conc of nitrate.
Diphenylamine in H2sO4 yields a blue oxidation product which you will be able to track the conc. of with the colourimeter, which is a cheap spectrophotometer.
The other way to track ammonia would be with it,s pH effect on the water it is dissolved in. Ph duino or for a simple yes no you could use a pH indicator such as phenolphylene. red/pink yes clear no.
If using pH remember that it is a logrithmic scale. So from 10-11 is a lot more alkali than 7-8.
depending on the physical form of your test if it is gas/atmosphere you could go old school and use a Draeger tube.
These are single use tubes (I think), they contain crystals that change colour depending on the conc. of ammonia or whatever the tube is designed for, in the atmosphere you are testing.
I assume you are running at a pH over 4 because if you were below 4 then your sodium nitrite would become nitrous acid and a) volatile and b) detectable on starch iodide papers.
Are you showing blue on congo papers? (if so sub pH 4)