Here's what I wore with them:
I did it on a pair of dark blue shorts, but I think it would probably work better on black shorts. I used normal craft acrylic paint on them (don't use washable paint, obviously) in black, purple, blue, white, and red. I wasn't really following a tutorial or anything, I just looked at a similar pair and got experimental.
First, I just kind of painted black splotches all over them. Then I just added a lot of blue and purple. Then I took the red and mixed it with a little bit of white and black to make it not so bright and did a couple swirls of it on top. Next, I mixed together some blue and purple and made splotches of that and then swirled the white into it. Finally, I took the white and made stars.
It dried pretty quickly, and overall I'm pretty happy with them!