River colors 3 and 4 indicate rapids. Waterfalls are River color 5. Ocean color is the color of the region surrounding the Elevation Model data. This color need not be the color of the sea or of water at all. Depending upon the effect desired, it might be green to match the lowest landscape colors.
Sky color is simply the color of the sky.
You can obtain an interesting effect by making Sky and Ocean colors black and by setting the Haze value to 0. This creates landscapes that appear to be floating in space. Sky Haze is the color that the sky fades to in the distance. You can produce another interesting effect by setting the Haze and Sky Haze to black and the Haze value to a high number like 300. This makes the landscape look like a night scene lit up by a light behind the camera.