I believe the edge is darker when using that technique because the edge dries first and then the surface tension of the water pulls the pigment that direction as it dries faster on the edge.

BTW, I don't know if you use Photoshop, but there are some fantastic natural media brushes from Kyle Webster. You can check out his work and brushes: http://www.kyletwebster.com

I do a bunch of visual chameleon work so I'm always keeping my tool set varied.