Since NO is generally thought of as a colorless gas, it would not have a direct color, but would generate a coloration through a similar process as our atmosphere (Rayleigh scattering). This process is based on the molecular size of the gas and is based on the wavelengths of the light (blue light has a shorter wavelength than red so is scattered less). NO molecules are similarly sized to our atmospheres major constituent N2 and O2. This would tend to produce a blue coloration. [This is a great simplification of the process.]
NO2 however does have a color, it is reddish brown (actually the leading contributor to smog coloration). This would limit your visibility through the atmosphere considerably producing a much hazier atmosphere and be a major contributor to coloration.
The combination of these sources should lead to an atmosphere in the dirty purplish coloration.
**I'm assuming a light source that is colorless, a redder or bluer star would change the color of the sky accordingly.