You can use any Sharpie colors. You probably want to make sure that any colors that touch will blend properly. I was careful to ROYGBIV all of my colors. If you draw purple and orange together, you'll get blended brown (which you may want- I don't know your life). Same with red/green, blue/orange, and purple/green.
Draw. Make them match. Or not. Up to you.
Use a pipette or spray bottle to apply undiluted rubbing alcohol to the shoes. Sharpies have an alcohol-based ink, so the alcohol dilutes and blends the lines, and softens the edges. The colors will start blending almost immediately. It's fun to watch.
Left is before alcohol, right is after
Prettiest shoe insides ever.
Let the ink cure for 24 hours, then rinse in cold water to get rid of any residual alcohol. Allow to dry.
Lace 'em up, and WOOT!
Sharpie ink is not water soluble, so my feet will not be tie-dyed if the shoes get wet.