Hey Handro!
I'm also a fan of your rendering, so it's a pleasure to answer these questions. My choice of colors for the rendering comes from the original painting (Hylas and the Nymphs, by Waterhouse), some reference pictures of lilypads, and finally a bit of something I picked up in James Gurney's book, Color and Light.
At first my water was a lot darker, but it moved iteratively towards a muddy brown. As for the reflections, they're basic shapes with a vertical motion blur, then sideways disturbances. Somewhat emulating this effect:
Looking at reference pictures is key when rendering - often water only needs to reflect its surroundings, so the sky's blue is not always appropriate!
