Sketch out the wave.
Now  fill in the water base with green, blue, and a little bit of red..
Continue to fill in the water.
Add in white to lighten the color for the wave.
Paint in the waves shape and clean up the water a little bit.
Mix in a little more white and a little green to further lighten the colors.
continue to fill in the wave also starting to blend the colors together to clean it up a little.
Mix more white to form the waves misty background.
Fill in the rest of the background and blend the colors a bit then start with white wave highlights.
continue to fill in the wave using light green and yellow to form the waves eye color, and a dark blue and deep red for the dark water highlights.
Using a mix of white and a little yellow highlight the waves face  and spray.
Continue to fill in the waves highlights.
Finish off with the water highlights.
Now stand back and enjoy your new tsunami wave.