I used to work a lot with MS Paint until I've discovered this amazing programs which allows to do all the things I did on MS Paint plus many other things.
Except... There is something I could do in MS Paint that I don't know if I can in RealWorld.
Basically, let's say you have, for example, black as your primary color and green as your secondary color; if you then use the eraser while pressing the right button of the mouse, the eraser will only affect the black areas, ignoring all the other colors, and re-coloring them green.
Now, my question is, can I do something similar in RealWorld?
None of the tools in RW apps works exactly as you describe. But perhaps there is a different way to accomplish your goal. If your goal it to re-color certain parts of the image, maybe you could try the floodfill tool. If you click on a colored area, it will fill it with the primary color (or pattern).