Here are my demonstrations from last week all of which were completed in my classroom in a 30-45 minute time window. I try to do a lesson on composition followed by a fairly complete painting. As you can see some work better than others but there is no redo in watercolor and no serious over painting like in oils. 
14 x 20 watercolor on paper the goal here was to use and preserve small pieces of white paper to move the viewer through the painting. I think this one worked pretty well.
14×20 watercolor on paper. I was trying to emphasize how much the simple addition of figures can really focus a painting. I was working with a very limited palette of cerulean blue, olive green, burnt sienna and cobalt violet. I tried to affect the feeling of shadow and light.
14×20 watercolor. I can’t really remember what I was trying to teach hear other than the quality of direct washes and good brushwork, which I guess is the goal of everyone of my lessons. I wanted to capture the light out behind the trees.
Here is the oil painting of this same image.
I like very much your 3 paintings. Have you use mask in the first one? I an beginner. How may I organize the white shapes¿? May I draw them in the sketch ¿
Mercedes, I always use the side of my brush and cut around the whites, this gives me a chance to get the edges I want. Try to use mask only on the smallest pieces of light and then you will get comfortable with preserving lights with only the brush.
Nice. Especially love the whites in the first, the mood in the third, and the rocks in the 5th!
Peg
Thanks Peg I hope all of your workshops go well this summer. Thanks for the comment
Thank you very much for answering me Steve. Now I realize why it is so wonderful your painting. Respectfully