This sort of falls under the "paint was you really see rather than what you think you see" school of lessons. Many beginner painters look at lakes, ponds, puddles and want to paint the water. That flat (or not) area that surely is blue, right? Nope!
Water is reflective. How much will vary on:
- the angle you're viewing from
- the water activity (still or churning)
- any materials in the water such as mud
So if you're a bit stuck on the colours you see in the water surface, then look at the sky. The water will always reflect the colours and tones of the sky to some extent.
Here's some easy ones to start:
First image, a clear blue sky with fluffy white clouds reflected in fairly still water. Then the same kind of weather reflected in water with bits of wave. So these need blue and white in the painting.
Here's a night example:
In the first image we see the dark blue tinge of just normal night light, with touches of yellow light from building windows (most normal lightbulbs emit a yellow wavelength of light). In the second, however, it is the reflected reddish lights from nearby buildings and Greenwich Pier reflecting on the surface.
How about a more subtle example? This is the same view in slightly different light and weather. First is a clearer day, second is an overcast day. Can you see the subtle difference? A stronger blue in the first one? (both were late in the day so there's a pinkish tinge from dusk approaching too)
Finally, about that angle of view. The first photo here shows a bit what I mean by the angle you view from and also what's in the water. While it was a bright day I was looking straight down, so didn't get a reflection from the light from the sky. The water is almost opaque because it's very near the shore and mud is being churned constantly by the waves - so this gives the water it's colour instead of the sky above. The second photo is from an angle looking outwards over the water, so there's much more reflection of the daylight on the water surface. In sea views this can be why the near water is a different colour than the distance - you're near enough the area where sand is being moved around creating an opaque area in the water while the further water is deeper without materials being churned around and your angle of viewing means you see the reflection of sunlight on the water. So sea paintings can have several colours changing as you move from shore to horizon.
So if you're ever in doubt, forget what you're looking at. Look up, see what the light is like in the sky, then look back at the water. Try and see where the light is reflecting strongly or not. Then start mixing those paints.
Hope you have bright sunny days!
All of these images can be found on my Wavemechanics photo project page, and all have Creative Commons copyright so you can use them to create your own paintings. While I haven't posted any new photos in a while I promise there are many on my harddrive just waiting for editing and adding later. :)