About 20 DSOs have been identified as voorwerp objects.  These are the only DSO objects which have a greenish glow.   

The Hubble site describes this phenomenon as follows:  
“A quasar beam has caused once-invisible filaments in deep space to glow through a process called photoionisation. Oxygen, helium, nitrogen, sulphur and neon in the filaments absorb light from the quasar and slowly re-emit it over many thousands of years. Their unmistakable emerald hue is caused by ionised oxygen, which glows green. These ghostly structures are so far from the galaxy’s heart that it would have taken light from the quasar tens of thousands of years to reach them and light them up. So, although the quasars themselves have turned off, the green clouds will continue to glow for much longer before they too fade.

The first object of this type was found in 2007 by Dutch schoolteacher Hanny van Arkel in the online Galaxy Zoo project (in galaxy IC 2497). This bizarre feature was dubbed Hanny’s Voorwerp (Dutch for Hanny’s object).”