Is it possible to draw on shape lines using a brush?

What I do is make a new layer and draw on that. Then you can merge the layers later. Paths, even with the freeform pen tool, never look the same as drawing with the human hand. If you need your drawing to be vectorized for some reason, you can make paths from it afterward and tweak them.

If you draw the shapes in Path mode and not Shape mode (options bar), then you can Stroke the Path with the Brush tool using the Paths panel — either the panel menu or the button at the bottom. It will use the current Brush settings and foreground color, so set those before applying the Stroke. The paint will go on the current layer, so pay attention to that as well.

If you edit the path, the paint on the layer will not change, but you can erase it and stroke the path again. Watch your layers.

If you have Illustrator, though, it does what you are asking. You can apply a Brush to any path, then redefine the Brush afterwards, change the color, width, edit the path itself, etc., etc., etc.

~ Jane

~ Jane

I think tha best you can do is to create a brush that varies the size, angle, and other aspects, then stroke the path.