Answer
The light would appear blue to an observer on the Earth.
Work Step by Step
In part (a), we found that the light would appear to have a wavelength of $420~nm$ to an observer on the Earth.
The range of the spectrum of visible light is from approximately $400~nm$ to $700~nm$
Light with shorter wavelengths near $400~nm$ appear blue, and light with longer wavelengths near $700~nm$ appear red.
The light would appear blue to an observer on the Earth.
