In the posting by Nightwatch "What is this green colored thingy" that started this all off http://www.galaxyzooforum.org/index.php?topic=8926.msg88388 Zookeeper Kevin says much the same thing as Fermats Brother.
The fact that the image is green comes from the fact that the powerful [OIII] line sits in the r-band (which is the green in RGB).
This makes no sense to my tired brain, as I thought red would be the opposite colour to how it should be. Very interesting.
Why do galaxies with such strong emissions appear red, when, say, a W-R galaxy is usually bright blue. Confused....Our sun
is orange-red, yet this does not have the enormous emission lines. I think I am barking up the wrong tree. It seems very
contradictory.
Hi Rick Nowell link - It might be useful for zookeeperKevin to comment on my postings (any moderators listening ?).
"the r-band (which is the green in RGB)" is incorrect. The r' filter is centered on 6230 Å, which is red in the visible spectrum (see posting 821).
To recap, the "powerful [OIII] line" "sits in the r-band"
only when the z shift >0.1 and <0.4 (approx).
"as I thought red would be the opposite colour to how it should be". This is what
you thought they should be !
But I guess that the astrophysisists wanted more information in their screen images and decided to "stretch" the normal visible colour range into the IR.
"Our sun is orange-red". To most people our Sun appears white !
"galaxies with such strong emissions appear red". Most galaxies are "white" (generalising) as they are made up of billions of stars like our sun.
They are basically black-body radiators, emitting a continuous spectrum (the peak and shape in the spectrum depending on the surface temperature).
If galaxies are orange or red, it's due to the "red shift" (z) resulting from their distance from us. The Sun would appear a different colour if it's temperature was different, or if it's z was different from 0.
Objects with associated emission lines (on top of a continuous spectrum) are surrounded by gas.
The gas is absorbing radiation (often UV or X-rays) and re-emitting at very specific wavelengths due to transitions between various electronic bands in energised atoms.
An appropriate example here is OIII, which is an oxygen atom with two electrons removed, emitting at exactly 5007Å (z = 0).
"Our sun does not have the enormous emission lines". It does have a multiplicity of absorption lines called Frauenhofer lines. This is where atom are absorbing energy from the Suns radiation in the photosphere. If they absorb, I guess that they must also emit.
See
Suns Spectrum.
Hope that clarifies a few points,
Fermats Brother 
