I like using acrylic wash because it has the look of watercolor but it doesn't reactivate when it gets wet. This way I can do all my values and modeling first in a way that can't get messed up, then I apply the brighter but much more delicate concentrated watercolor as the final step to add all the chroma.
As you can see below I start adding the concentrated watercolor to her skin, then bathing suit next. I do the light colors first because if I did it the other way around there's the danger of the dark color bleeding into the lighter. At this stage I don't attempt any modeling, I only lay in the concentrated watercolor as flat tones. Modeling will only mess it up. The Lumas are completely translucent so the acrylic washes underneath show through perfectly. Next I move on to the orange of her hair and the blues for the fish and the ocean and sky.
I always paint a test subject simultaneously. I try my colors on the test first and if it looks a little off I can fix it for the real piece.
And here's a scan of the final art.