There are many ways to add color to the tail of your mermaid doll. Painting with Genesis heat-set paints is easy. You can add and bake a very thin layer of Sculpey clay in a color you like. You can add a thin layer of MixQuick or liquid clay. Glitter is often sprinkled onto the tail too.
Here is another way of adding color to the tail of your mermaid doll; check it out:
Several options are shown here: http://www.sculptuniversity.com/index.php?option=com_content&view=article&id=121:mermaid-tails&catid=38:freebies&Itemid=151
Just experiment until you find the look you desire.