It's really intimidating to paint underneath something, knowing that you're going to be painting over it and leaving only parts exposed. It's hard to tell exactly what it's going to look like. Masters like Gerhard Richter could do it effortlessly, but the rest of us poor schlubs have to hope for the best.
Unless... unless you have photoshop and can test it out a bit. Below is the undercoat and pattern that will be going over it. The black will be exposed, and the white will (ironically) be painted black. I'll come back with the Photoshopped version when I get it done!