Black 'Dimensional' (3D) fabric paint. You just squeeze it on and wait for it to dry. Pretty simple. You can also do it with hot glue, but that can melt the foam. The 3D paint you can probably get at your local craft store (like Michaels). I use Tulip Slick Dimensional Fabric Paint if you care.
Cool. Good luck, I can't wait to see yours. Mine should be done by Monday depending on the weather.