Why does the color of Arcturus oscillate from red to green through a telescope?
This is due to the Earth’s atmosphere acting as a lens to refract the light from Arcturus into a mini-spectrum. Blue, green and red light refracts to different degrees and your images will be colored as the transparency of the air column you are looking through changes due to turbulent eddys at high altitudes. This effect is most noticeable at low elevation angles near the horizon, or on nights soon after or before a storm system has passed through.
