Thought I'd post a quick example of how value makes the painting. Value, or dark and light, is what holds a painting together.
Here's a recently finished oil painting done from some studies along the American River:
I started this piece with a neutral underpainting value study. I first quickly toned the canvas with a bright yellow rubbed out with gamsol thinner. I then painting with a thin purple/yellow mixture that is fairly neutralized (but not a completely neutral grey). This resulted in a monochromatic painting that lays out both the complete composition and value (dark/light) structure of the painting:
Notice how monochromatic does not necessarily mean black and white (nor grey). You can paint monochromatically with any single dark color. It just means one (mono) color.
It's interesting to compare this underpainting (above) to a black-and-white version of the completed painting (below).
When you compare the monochromatic and B&W versions, it's obvious there is a lot more color in the monochromatic underpainting than you might first think. By starting with a value study, I find I can work out lots of problems and design pleasing paintings before even starting to consider color. This is a great way to simplify the painting process.