Due to Krypton’s large number of spectral lines

its ionized gas is white, which is why lightbulbs that are krypton based are used in photography and studio lighting in the film industry. In neon lights, Krypton reacts with other gases to produce a bright yellow light as well. The isotope 85Kr can also be used in combination with phosphors to produce materials that shine in the dark due to the fact that this particular isotope of Krypton reflects off of phosphors.

Source: Chemistry of Krypton, ChemWiki: The Dynamic Chemistry Textbook,  University of California, Davis