Radon is an extremely unstable noble gas. Due to this, putting it in a gas discharge lamp for a prolonged period of time is extremely hard, as pure radon is needed to give a true color, but radon decays, contaminating the lamp. But suppose we were to find a way to not let radon decay, concentrate it in a gas discharge lamp, and turn the lamp on. What color would the radon gas glow?
Sources I find are contradictory. images-of-elements.com says
Radon is said to glow red in discharge tubes
and the image that it gives
is only an illustration, not radon itself.
Wikipedia says that it "glows green or red in discharge tubes", but it doesn't cite any sources there.
The spectral lines of radon are listed in physics.nist.gov/PhysRefData/Handbook/Tables. According to this source, radon gives probably somewhat reddish due to the strong lines at 705 and 745 nm.
Radon glows violet/purple when discharged. The reason why is simply we can infer it off of the patterns of the periodic table. You see, radon is at the very bottom to the right, in the noble gases column. Noble gases are already stable (for the most part) and do not want to react with other particles; thus requiring a lot of energy to react. As radon is the largest natural noble gas element, we humans can mess with it a little bit more than its other noble gas brethren. Now since we know it requires a lot of energy to react, we know it is related to light with an insanely short wavelength (shorter wavelength=higher energy). If we look at the light spectrum we can see the shortest wavelength and highest energy wavelength is purple/violet/indigo. Thus making radon that color when discharged.
And based on the emission spectrum from chemistry.bd.psu.edu, it appears that radon would fluoresce with a deep bluish-violet color.
Same question for oganesson. Although Wikipedia says it should be a solid due to relativistic forces, what color would oganesson glow in a gas discharge lamp?