I was watching a video about nitrogen and started to think what would make nitrogen react by showing signs of color on the physical side of things without being harmful to surroundings. And this would be in a world that doesn't have nitrogen in the atmosphere. What are some chemicals that can achieve this when combined with nitrogen?
Unless your world has very different temperatures and pressures from Earth's, there's not much you can do. From Wikipedia:
In general, nitrogen is unreactive at standard temperature and pressure. N2 reacts spontaneously with few reagents, being resilient to acids and bases as well as oxidants and most reductants.
Microbial Biochemical Assay
One way might be using organisms like nitrogen-fixing bacteria grown and isolated in a nitrogen-free environment, and analyzing their morphological characteristics and pigment production. For example, you might have an inoculated Ashby-Benzoate agar plate that does nothing and appears plain until nitrogen is introduced into the environment (through the air, titration into agar, etc) and those nitrogen-fixing bacteria spring to life, producing easily detectable morphological and biochemical markers. The plate may suddenly start producing a multitude of small, white/cream, round, 2-3mm, glistening colonies or the agar substrate reacts by changing color.
Another possibly with another chemical like Benzoate that reacts with specific pigmentation produced by the bacteria to visually show the presence of nitrogen through the inference that the nitrogen-fixing bacteria could not be present producing the pigments to react with the Benzoate if there were not nitrogen present for them to fix non-symbiotically.
Possible forms of Nitrogen
If there is no nitrogen in the atmosphere, then there will be very little nitrogen present as N$_2$ (otherwise it would be in the atmosphere). Therefore we need to take a look at other potential forms of nitrogen and how we would detect them. The detection method would be different for different forms of nitrogen. In a biologically active world, the three main nitrogen compounds (using Earth-based life) will be ammonia, nitrates, and nitrites. They are associated with each other through the Nitrogen cycle. I will go over them, and some other relatively common nitrogen compounds.
One of the most common forms of nitrogen, especially in the outer solar system. Ammonia is a gas down to -33C, and a liquid to -78C. A simple ammonia test uses Nessler's reagent, which is 0.09 molar potassium tetraiodomercurate(II) in 2.5 molar potassium hydroxide, which will turn yellow in the presence of ammonia.
A common anion that forms many salts. Tested for with the nitrate test, where iron(II) sulfate and sulfuric acid added. If nitrates are present, they will be suspended in a 'brown ring' between the layers of aqueous iron(II) sulfate and sulfuric acid, indicating nitrate presence in a test tube.
Another common anion. Tested for with the nitrite test. This is very similar to the nitrate test, except that the test solution has 4M (less concentrated than in the nitrate test) added to it until acidic, then iron(II) sulfate. If nitrite is presence, the whole solution will turn dark brown
Amines are ammonia molucules with one or more hydrogen atoms replaced with a substituent, such as an alkyl (a hydrocarbon chain like methyl, CH$_3^-$) or and aryl (a hydrocarbon ring like phenyl C$_6$H$_5$). Amines are of course important as amino acids, but there are a great variety of possible forms. Amines are detected through the Hinsberg test; where the test solution is mixed with sodium hydroxide and benzenesulfonyl choride. The amine will form a sulfoamide salt which will precipiate out of solution and indicate amine presence.
Toxic and reactive compounds, used in things such as airbag inflators. Not commonly tested for, though there are some papers detailing detection methods
N$_2$0 is nitrous oxide, or laughing gas. NO is nitric oxide. It is poisonous in large quantities by is used in the body for signaling, most importantly by causing erection. This is the 'active ingredient' in Viagra. NO$_2$ is nitrogen dioxide, which is a major air pollutant (this is what Volkswagon just got in trouble for with their diesel engines cheating on emissions tests). It is a precursor to nitric acid and a major component of acid rain.
These oxides are all gases and cannot be detected as easily using the liquid-type tests for the previous compounds. I would guess gas spectroscopy would be the easiest way to identify the presence of these compounds.