c. denotes using the 'BRUSH' setting (on separate object/layer) with the 'MERGE MODE' set on 'ADD' (see arrows - above) . . . . note: the 'background' line work does appear/show-thru . . . . . . but does NOT run-over the applied brown coloring . . . and only the line work is colored/shows-thru, which is what I chose for this drawing . . . . .