The Smudge and “Smooth” (ALT+LMB) tools are of no use right now for those colouring in an artistic way.
+1 Smudge Tool needs some love someday 
[deleted by the author]
[deleted by the author]
Image created with Pencil2D
This image was lost from this post (http://www.pencil2d.org/forums/topic/my-drawings-in-pencil2d/#post-4862).
It was created by the year 2014, in which the Smudge (improved) and Smooth (new addition)'s tools were improved by @Jose (I think he is not here any more), and I felt that developing them both a little bit more would allow to “paint” resembling traditional paintings (watercolour, for instance).
Of course, Pencil2D is targeted to animation purpose, but it doesn’t make any bad to have it capable of doing better work than this one)
Currently those tools (smudge/Smooth) are not able to give 20% of the end result of this image, that is why I pasted it here for future comparison.