This one really was done in the spirit of the daily exercise.. about an hour's time total, with concentrated effort of making each brushstroke right. I am pleased with this one, even though its just a simple scene and small (5x7 because I'm out of my 8x10 panels).
The fun thing about painting something white, especially something white where there is lots of light bouncing around.. is seeing all the color in it. You can intelligently deduct some of these color even if they are not real obvious in what you are looking at. As mentor Stapleton Kearn's says, "it's not WHAT a painting is about, but HOW it is about that makes it art". So, enhance some of what you see to make a beautiful statement.
In this case, the sunlight is warm, so everything in direct light is going to be a warm white ( I used cad yellow light and white)
Everything in shadow is a cooler blue, but, because of the surrounding elements, some of the color of the water, etc. can't help but creep into the shadows, so it's got a bit of warmth in it too. I used a blue with a whisper of an earth red -
The reflected light is where it really gets fun. Things on the bottom of the horse, where the sun is hitting the water and then bouncing back up to the horse, will be warm. Areas of the horse where his form turns upward towards the sky, will have sky color reflected, so you'll see some stronger, purer blues there.
Hope that gives you some ideas to try on your next painting... I'm planning a larger work with this same horse in it . We'll see how that one works out...