well, I like bright coloured nebulas, and If you browse through "real" star photos you can find even brighter and more coloured nebulas.
But the point of the thread is...don't you like em? then do it by yourself in the way you like.
Images from outer sourcers, such as greg martin's nebulas or "real" photos, simply are not suited for a ingame background: you can't use many of them due to copyright, most have edges, or suffer of some grain/resolution problems, or they simply look odd once in game.
The images don't simply have to be nice, they have to be nice in game.
And since it isn't that difficult, with some practice, to create an image from scratch close at the level of "real" photos or images created by other artists (this one is my first attempt ever at a starfield...), the best solution probably is: do it by yourself designing your image specifically to be used in game.