With the rise in popularity of the Boost line from adidas, new ways of keeping your Boost looking fresh have come with it. The SneakersER midsole paint pens use an acrylic flexible paint which covers up the dirt and yellowing of the Boost. First give your Boost a good clean to remove all top-layer dirt. Then follow on with a generous layer of paint for your first layer. Let it dry for at least 1-2 hours before applying subsequent layers, and soon you will have brand new Boost!
If you're looking to custom - check out all the other colours available here.