It's about keeping it simple: black and white only, which of course are RGB(0,0,0) and RGB(255,255,255). If you want to use colours, you need a colour selector which is unnecessary fluff at this point. Obviously it'll be a critical element fairly soon, but right at the start of the project it's (arguably of course) less important than the application structure and just getting an MVP together. If you want to do colours before you do File-Open and File-Save that's a valid choice.