I designed it to be like that so you can only see the cutie mark if you are looking for it. There are 2 layers. Each layer is truly 3 compacted layers. In the cutie mark layer there are lines going /,\, and |. The same goes for the background. They are all generated on the same seed so they cross over in the exact same places. On each of the 3 cutie mark layers there is a rainbow gradient that corresponds to the exact values of Rainbow Dash's mane from left to right. I then changed the opacity to 50% to blend the cutie mark into the background. I will upload a picture of it with increased saturation.
This was done 100% digitally in Paint.Net. I can't really retake it but I can modify it. It was originally this picture. http://fc09.deviantart.net/fs71/f/2012/198/a/b/rainbow_dash_cutie_mark_by_missmoo10-d57mdcs.png
It took me awhile to get the picture to a 1x1 pixel out line but once I did that it was as easy as magic wanding and using special gradient plugins.