Do green stars exists? I asked a university lecturer why we don't observe green stars, and he said the blackbody curve averages at that frequency such that the cones in our eyes don't recognise it.  I have a hunch that white stars emit shades of green, and this is reflected by chlorophyll, but this paper throws confusion on the matter "AEGIS: THE MORPHOLOGIES OF GREEN GALAXIES AT 0.4 < Z < 1.2"  http://arxiv.org/pdf/1101.3353.pdf
What is green about 'Green Valley' galaxies, if anything?  My hunch is that they're called green to emphasise the fact that they're some kind miscellaneous class, i.e. not particularly white and not particularly yellow, just somewhere in between blue and red.
 A: The answer given by user28161 is perfectly correct as to why there are no green stars. In fact, go and look at the stars some night (with binoculars - too few photons and you lose color sensitivity) and you'll notice all sorts of colors but a noticeable lack of green. The colors you see are pretty much exactly the colors you'll see when metals are heated to very high temperatures, since the same physics underlies both emission phenomena.
As a footnote regarding terminology: The "green valley" is the region in parameter space between "blue" galaxies who are undergoing bursts of star formation and "red" galaxies with only old stars. While the details have yet to be worked out, we believe that whenever galaxies (or large parts of galaxies) undergo bursts of star formation, they produce roughly the same distribution of stellar masses (the "initial mass function"). As luminosity scales superlinearly with mass, the light output will be dominated by the largest stars, which will also be the hottest, with spectra peaking in the blue/UV.
However, massive stars last only some millions of years, as opposed to tens to hundreds of billions of years for very small stars. So after the star formation has ceased, the big stars die, leaving behind smaller, redder stars. Most galaxies are either in the blue region or the red region  of parameter space. If you plot color (or better yet, star formation rate, which is extracted from color together with dust models and stellar population synthesis) versus the total mass of stars in the galaxy, you'll see a clustering of points along these red and blue sequences (with the redder galaxies tending to be more massive). This is done for instance in Figure 9 of Wyder et al. 2007, APJS 173 293, where "color" is taken to be the difference between near-UV flux and red flux.
Some galaxies, however, are in the "green valley" between these regions. It is still being debated whether these are indeed transitioning from "blue" to "red," or if they are part of a small minority of galaxies that spend most of their time as "green."
In any event, as mentioned in the aforementioned answer, spectrally "green" is more like visibly "white" to our eyes.
A: If the majority of the radiation emitted by a star is infrared, the majority of the visible light emitted will be red.  We don't see the lower half of the spectrum the star emits.
If the majority of the radiation emitted by a star is infrared but some is visible, the average visible light emitted will be yellow. Little blue is emitted and any green averages to look yellow.
If the majority of the radiation emitted by a star is high energy, the majority of the visible light emitted will be blues.  We don't see the upper half of the spectrum the star emits.
If the average wavelength is right on the green wavelength, blue and yellow and red light will be emitted as well making the light from the star look white.
Below is a similar explanation from someone much smarter than me.
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970408e.html
