Colours make the difference. Colours are not the same. Neither for galaxies.
Galaxies can be of various colours: purple, blue, red, white, orange and green.
Yes, also green. But these colours have not the same probability of showing.
Green in particular is not so easy to obtain…
Galaxy colours, indeed, reflect the galaxy (and its star) origin and evolution. Star age and evolution “mean” a specific colour. Whereas red and blue galaxies are relatively common, green galaxies are pretty rare because such a colour reflects a specific stage of their evolution.
A green galaxy is a galaxy that is “rapidly” (term to be always intended in the “universe scale”) changing from blue (a galaxy stage where stars and planets are forming) to red (a galaxy stage where stars begin to burn themselves out). Green is actually that colour showing the turning point in the galaxy life: young and tumultuous are blue, old and lazy are red. In the transition… green.
“This suggests that the few green galaxies we catch are likely to be at a critical stage in their evolution; rapidly turning from blue to red.”
During the galaxy evolution the “green stage” can take place in two situations. On one side, if the green galaxy is rather small (e.g. a Pea galaxy), it is very likely violently tossed around by the gravitational pull of a massive neighbour, thus their “green gas” s stripped away. On the other side, bigger green galaxies may undergo to self-destruction caused by super-massive black holes at their centres that can blow dense gas away.
This is the result not (just) of direct observations, but of time consuming (although very accurate) computer simulations (relying on the EAGLE simulations developed by the Durham University’s Institute for Computational Cosmology).
The realism level of our universe (and our guess about its evolution) is coded in some bits and the colours these bits reveal is in continuous evolution. Passing through the green.