I originally made the PDFs (a document) for printing out and coloring by hand and the PNG (a graphic or picture) for coloring in software. While converting the PDF to PNG is easy in a Mac, I had a little problem. The image is “dithered” which causes that light-colored, thin line between the gray edge and the pink interior in the picture below. It's probably just me, but I don't like the look.
Rather than go through long explanations that would cause your eyes to glaze over, I decided to record the keystrokes in a video of my computer screen. Being a geek, I know there must be an easy way to do this. There is using QuickTime. In the video below, I have opened the PNG in Paintbrush. I open Paintbrush's toolbox, click the paintbucket icon, and choose the color black. All I need to do is click one portion of the gray in the image and all pixels connected to that one turn black. I picked other colors and started filling in diamonds.
Moral of the story: If you think there is a way to do something, there probably is. Geeks figure out how.