My method is a bit more quick and dirty, but here it is:
Step 1
Choose your colour, mix it to near black
Step 2:
Paint a cloudish shape on your base
Step 3:
While the paint is still wet, mix in a slightly less black paint mixture towards one edge
Step 4:
Mix in an even slightly less black mixture towards one edge
Step 5:
Keep doing the rough gradient towards that edge
Comparison shot: You can see that the dark grey isn't black enough, so I'll fix that.
Step 6:
Water down some black and paint it on. For a better fading effect, I just used my finger and smeared some black gently towards the nebula.
Which should make it look something like this:
Step 7:
Just add some cloudy edge highlights. Then, using the same colour you usef for the edge highlight, water it out, and apply to your thumb. Then, starting from the bright edge of the nebula, rub it on very lightly to create a thin haze. If you botch it and the haze is too thick or not in the right place, apply water to your other, clean thumb and rub it off before the paint dries. You should get something like this.
Step 8:
Take a course bristled brush, dip into some watered down white, and flick it gently at the base -- make sure you do it from directly above the base, you don't want the paint droplets coming in from an angle. Do a test run on some paper to see how much force you need if you haven't done it before.
Step 9:
There is no step nine. Behold, His Emperor's Ships the Belligerent and the Conspicuous, floating on a cloud.
