Summarize this article:

On the bottom or outsole of the shoe, run a wipe between the rubber grooves.  Take a fresh wipe and gently rub along the edges of the boost.  After you use the wet wipe, gently dry the boost by wiping the edges down with a paper towel. Any wet wipe will do, although you may want to use one with antibacterial or stain removing properties to help remove dirt. If the marks don’t respond to a wet wipe, a bleach pen may be able to lighten the stain. Take the cap off and rub the marker over the stain completely. For the best results, put the shoes in the washing machine afterwards. You can get paint pens and oil-based markers at craft stores. Take off the cap and gently run the tip over the stained area. You may need to go over the whole boost so that the entire sole is the same color. Let it dry for a few hours. Paint pens and oil-based markers can give off fumes. Make sure you are working in a well-ventilated area. If you start to feel dizzy, take a break.

Summary:
Run a wet wipe along the bottom and edges. Use a bleach pen on dark or stubborn stains. Hide permanent stains with a white paint pen or oil-based marker.