How to color a mermaid

Here I have started with line work with this image I wanted to keep colors clean and simple

Once the line work is created you will want to add a base color. This will be your chosen color for the design

Finally to add more depth you will want to add shadows. Simply go into the layer your using and click multiply. Now the color you used will be made much darker. 

After that is done you can mess with the opacity settings  to get your desired look