Using a wet-on-wet technique, I had some fun with these mermaids. I would paint a section (like the tail) with just water to wet the paper, being careful about my outline. Then I would load up my brush with plenty of wet pigment and touch the edges of the wet area. The ink spreads out from there, creating a nice gradient.
You can see between the first and second how the ‘scales’ on the tail softened out as the pigment spread and dried. To stop this happening, apply the scale effect after the area has dried. I waited until it had dried only a little bit as I wanted the softer effect.
The final picture of both is down below: