What make-up techniques could be used to achieve the watercolor effect shown in this image?
I suggest a basic sunless Tanner foam ( lightest tan)dispensed mixed with some color agent that won't dye the skin permanently, like a children's bathtime paints. 
   Or, possibly use party streamers then either get wet to apply(an oil perhaps) or spray with body glitter for eye-catching gleam! 
   Good luck!