How to draw mandala when I intend to color it with pencil colors?
I can only think of a few options.


Don't worry about masking or hiding the sketch. Make bold, dark outlines on purpose. Sketch the pattern you want in graphite, then draw the lines very dark (in graphite, or even ink). This does give the final result something of a "coloring book" feel, which may not be what you're wanting.
Sketch outlines in the color of their section, or in some very light color (e.g. yellow, pale blue), instead of with graphite. This requires more planning of what colors will go in what part of the pattern.
Draw the outlines dark and bold, and put a second sheet of paper over top to trace those dark lines in the lighter colors of the final drawing. This is easier if you have a light board, but still possible even on a tabletop with sufficiently thin paper.