Well, It looks like you fill in the color, then use the glow effect from paint.net.
i used the fill can, (i do this for pictures with black lines unless im not in a rush, then i take my time like other pictures), then i take the brush, heighten the alpha transparency, and swoosh it around, then i brighten it around the middle. to make it look smoother, i copy the page, use gaussian blur, remove the glow around the body, then use a transparent gradient to lower it's blur a bit. its usually a desired effect, it kinda looks like if paper mario were in a glass hd thing
