Steve Willner <swillner@cfa.harvard.edu>

The color vision of our eyes is a pretty complicated matter.  The
colors we perceive depend not only of the wavelength mix the eye
receives at a perticular spot, but also on a number of other factors.
For instance the brightness of the light received, the brightness and
wavelength mix received simultaneously in other parts of the field
of view (sometimes visible as "contrast effects"), and also the
brightness/wavelength mix that the eye previously received (sometimes
visible as afterimages).

One isolated star, viewed by an eye not subjected to other strong
lights just before, and with very little other light sources in the
field of view, will virtually never look green.  But put the same
star (which we can assume to appear white when viewed in isolation)
close to another, reddish, star, and that same star may immediately
look greenish, due to contrast effects (the eye tries to make the
"average" color of the two stars appear white).

Also, stars generally have very weak colors.  The only exception is
perhaps those cool "carbon" stars with a very low temperature---they
often look quite red, but still not as red as a stoplight.  Very hot
stars have a faint bluish tinge, but it's always faint---"blue" stars
never get as intense in their colors as the reddest stars.  Once the
temperature of a star exceeds about 20,000 K, its temperature doesn't
really matter to the perceived color (assuming blackbody
radiation)---the star will appear to have the same blue-white color no
matter whether the temperature is 20,000, 100,000 or a million degrees K.

Old novae in the "nebular" phase often look green.  This is because
they are surrounded by a shell of gas that emits spectral lines of
doubly ionized oxygen (among other things).  Although these object
certainly look like green stars in a telescope---the gas shell cannot
usually be resolved---the color isn't coming from a stellar
photosphere.