Color intensity is important for conveying changes in the quality and location of light sources in a composition and in atmospheric perspective. Here's a good exercise for learning about color intensity from the introductory painting class I just finished with Lucas Graciano. We were given a landscape photo and told to paint it as a high, medium, and low intensity image without changing value. There are 2 ways to do this. First, mix the appropriate color, then add either the complementary color of the same value or a neutral gray of the same value. I opted for the neutral gray approach, using a set of 4 grays of increasing value that I pre-mixed from ivory black, titanium white and small amounts of raw umber (to balance the coolness of the ivory black).
The top left image above is the original photo. Next are the high (top-r), mid (bottom-l), and low (bottom-r) intensity paintings. I wasn't trying to render, just get a decent read of the big shapes. In hindsight, the low intensity image should probably be less colorful. I followed Richard Schmids' method, described in Alla Prima. He mixes the paint for each brushstroke based on the color, value, and intensity of the adjacent shapes. Once applied, if the stroke/shape is not correct he removes it immediately. His advice: Never leave a mistake on the canvas.
You can learn a lot from this type of study about intensity, and also about color mixing in general. Time well spent if you're just starting out. So, how did I do on maintaining the values? See the grayscale conversions of my paintings below. Not too bad, could be better. I'll work on it.
In his blog, All The Strange Hours, David Rourke posted a useful article on color and color mixing, with a great section on intensity, which he calls chroma (about half-way through the post). His blog is worth visiting, lots of good posts.