Add a transparent blue layer.
Add a light green background layer for the tail.
Add a transparent layer of grey.
Color on top of the blue layer on the body.
Color on top of the light grey layer on the fins and gills.
Vary the size of the spots to create texture.
Smooth the colors by coloring on top with a white pencil. Add black shadows on the edges. Finalize with a few reflects using a white or ivory Poska pen.