Re: Why do some stars appear to change colour while others do not?

Stars will appear to shimmer or twinkle due to the motion of the 
atmosphere. This is called scintillation. Planets do not apppear to twinkle 
or change color unless the atmosphere is very unstable. This is most 
evident when the stars are close to the horizon and less pronounced as the 
stars near zenith. The reason that this happens is stars are so far away 
that they have no apparent diameter when viewed from earth. They are points 
of light. Movement of the atmosphere causes the light to refract as it 
enters the air from space. As the density of the air changes between you 
and the star, the colors will change. If there is more dust and particles 
in the atmosphere, the color will tend toward the red. As a star moves from 
the horizon to more directly overhead, you are looking through less air so 
the star's color is more stable. 
Planets on the other hand are close enough to us that they have a visible 
diameter. Looking at any planet through even a small pair of binoculars 
will show a disk instead of a point. Because of this size the atmosphere 
has less effect on the light from planets. 

Here is a site that talks about the motion of the atmosphere affecting the 
color of stars.
History of scintillation