I use a highly advanced technique that consists of selecting a few colors typing a whole lot of digits through the 3D Studio MAX interface.
Actually, it's that, but combined with a multi-layered composite material normally involving four to six layers of the RGB Tint, Noise, Cellular, Perlin Marble and other procedural materials to achieve a nebula-like animatable fully environmental background effet.
I'm starting to develop a new technique for scenes ambiented inside
the nebula, this one involving a high number of atmospheric gizmos and low-density combustion effects. It's also fully animatable.