[MX] 'Graphite'-style line fill?

This is my scenario: I have a pencil that uses the simple drawing feature. When the mouse is pressed and dragged, the pencil draws a line. Basic stuff. What i want to do is have the line be a ‘graphite’ type of color, instead of the boring black, red etc. I’m thinking that i would use a mask with the graphite color layer on the bottom, and a blank white layer on top, then when the line is drawn, the pencil is actually erasing the top layer, to reveal the bottom graphite layer. :sigh: This seems somewhat farfetched.

What would be the best approach at achieving this ‘graphite’-style line fill?
Thanks for all that took the time to read this. Take care.

Victor.