For the colours, pick a few light, pastel shades. Mint (of course) makes a good base - white makes the colours look to drastic. It's a good shade to pile colours on, or you can leave it exposed. Another tip is to not dab all your colours on at the same spot. After the mint base, perhaps dab the orange on slightly to the left, and the blue, just a blotch in the middle. I added pink because I prefer a warmer look, and I topped it off with white stars with my dotting tool (you can use a bobby pin/toothpick, which will do the job just as well). I also added a sheer layer of glitter, to give it a little more depth, and sealed it in with seche vite top coat. 

The nebula nail is a design that never fails to look good. If your design isn't turning out the way you want it to be, just hit google and stare at pictures of nebula for a few minutes till you find that perfect combination of colours you love. If you've got any questions, drop a comment here or on my YouTube page - either way, I'll reply with the best answer I can give. Now go on and nail it good!