Using one color and the lighter and darker shades of it, you can create a rainbow-like galaxy-like picture that looks really cool.
Put large dots or stripes of ONE color only around. Don't put too many, this makes the drawing too loud.
Use the lightening tool at about a 20 size pen and lighten only part of the spots or stripes. In your drawing, you might not want to use too much light for this kind of wrecks it in my opinion.
Using the darkening tool, darken part of or more than half of the spots/stripes. It might change color--this purple turned blue--but that's because it was more of a bluish purple.
Use the blend tool to blend the colors together. This makes them easier to work with--different shades, but still purple.
Using the smudge tool at about a 20 size, make small C shaped motions across the whole drawing in a row. Make them even. This is a zoom of a couple rows I did.
Do this to the whole drawing--it will turn out to look like this. I don't know if you can see the text, but it says that there is a hint of yellow. Actually, it's more of a yellow-grey, but in the end, this will be quite obvious. This is what I like about these drawings--you start off with one color and that color only, and end up with its opposite.
Use size 40 of the smudge tool and do this step over again, still in a C shaped motion. This time, however, make the motions smaller--this makes the drawing less choppy.
Now use a larger brush, maybe a 60, 70, or 80 of the smudge tool, and making even smaller motions, still in rows, do this to the whole drawing.
Now do what you want with it--drag each color, blend it a little bit, etc. It'll look like a galaxy! Try to get rid of the effect made by the smudge tool, or the remains of the C's. I bet you can't see the text this time either, but the top one says yellow, which is a lot more apparent here. The bottom left says a pastel rainbow, since you can see a light purple, light blue, and light red/yellow there. The bottom right says orange--somehow, there is orange there.
And voila! You have a strange looking galaxy type thing, or whatever you want to call it!