Someone truly dedicated to a writing career will see these things as challenges to be overcome, and know that even though it will take a lot of hard work, they can indeed achieve their goals. So cheer up, put your heart and soul into your work, love the process and have faith in yourself. The simplest is to color in an area with a white pencil, then use colored ones. The paper will hold onto the first color that's applied, but subsequent colores are easily removed. So put the white down first, then color over the white.
Some people have great luck with electric erasers, just be sure to use a white eraser.
Colored erasers leave a color on the paper. For a tough spot use a very fine grit sand paper. Remove most of the color, then use an electric eraser or hand held eraser, like those pencil shaped ones that you push the eraser out of. Along these same lines, for a small area use an Exacto type knife to remove the color, then carefully erase the spot to remove any remaining color. This is unusual and there is the potential for people to not understand what they are agreeing to.