I love galaxy print, so I tried painting some black cutoff shorts with acrylic paint. It turned out well, and doesn't fade after washes. I used purple, blue, metallic blue, and white acrylic paint. Fabric paint should work just as well.
Dip the sponge brush in some paint and begin painting your shorts with the blue and purple paint.
You can blend the colors a bit to make it look like a galaxy. Once the paint is almost dry, apply a few more coats.
To create the stars, I used an old toothbrush, dipped it in white paint, and flicked it.
Let dry overnight, wash, and then they are ready to wear.