EDIT2: IF YOU ARE UNABLE TO COLOR THE BASE:
Go to Image > Mode > RGB Color. This option should be in both Photoshop AND GIMP, and it should allow you to color the base properly.EDIT: [link]
A Paint.NET Tutorial has been created to used these bases!
Thank you, ~Amabit!
Tutorial requested by
Earth Pony/Unicorn [link]
Hope it helps. :3