Paint inside, behind and on the color only

With the brush (B) or the pencil (Y), open the Paint mode button in the tool strip. The button’s icon shows the mode in use.

Paint inside the closed area keeps each stroke in the area where it starts, with nothing chosen first. Paint inside the shape uses the shapes that were selected with V when you turned it on. Only one of the two is on at a time.

Paint behind the artwork puts the stroke under what is already on the layer. Paint fills only takes out of the stroke whatever would fall on the lines of this layer and the layers below, vector or painted in pixels, and the lines stay over the color.

The Paint lasso (L) draws a free area for the brush and the pencil. Shift adds another loop, Alt takes a piece away, a click clears it and Esc drops a loop halfway. Invert area lets the paint land only outside.