I played a little more and really found the fastest way you can bring the color back out.
After you subtract your background,
1 go into the "colors" drop down
2 go down to "components" and select "channel mixer" from components
3 bring up the "blue in the blue channel" to 2.0
4 bring the "red in the red channel" to about 1.4
5 bring the "green in the green channel" to about 1.3
6 press okay
7 go into the "colors" drop down and open "saturation"
8 move the saturation to about 3 or 4.
Voila, you have color!
You can change the values to taste, but that gives you an idea of what to play with to bring the color forward out of the channels.
I will mention that you need to boost your blue first, then balance in the red to get about the right amount in there, then balance in your green to bring the stars back to a white color and the core of the galaxy to a golden color. There may not technically be any discernable green in galaxies but the green channel does a lot of work in giving the image depth in its details.
If you boost saturation before you make alterations in the channel mixer you will have some odd results. Try to only boost the saturation after you have tweaked the channel mixer to your liking. If you didn't do enough green after you saturate, undo the saturation and channel mixer and add more green, then redo the saturation.
One last note, you can bring the colors forward a little more by going into the "colors" drop down and selecting "hue-chroma" and boosting the chroma level. A little goes a long way with the chroma and I suggest you do it on a masked layer to keep the background under control.