i did the base neb in spacescape, then in gimp I added a top to them, the bright "baby stars" in the nebula, some dark nebs for shading, and the starfield.
I don't like the billboard stars in scape, i get much better control of them in gimp, ie the number of stars, the size of them, saturation. Same with the dark layers, they come out much better in gimp, more random, and with a greater degree of control. Random but with a greater degree of control, that statement sounds like a contradiction...but you know what i mean.
a few other modifications, a starfield in gimp, once again i like the amount of control that gimp offers me....and it is done.
As far as coloring goes...the base nebs from scape remain intact. The only coloring i did in gimp was to the layers i created, and even then, the bright stars layer was colored by the neb base layer gotten from scape.