Value in a painting, refers to the lights and darks that are used. Currently, I am working on a piece that I did some plein air painting on this summer, and took several photos of (it is not yet done, as I am just getting to the foreground reeds now). As you are working away sometimes you know that things aren't 'reading' exactly right, so it is always helpful to get rid of one thing to simplify the process. So... getting rid of colour by putting your photo in black and white and then taking a photo of your painting in black and white is very useful. 
NOW... it is easier to see that the background mountains are a little too mid-value when they should be a bit lighter value and probably a bit softer and fuzzier. The closer trees on the edge of the lake could go a bit darker too. You can almost paint this in any colours you choose and as long as the values 'read' correctly it can be a very interesting painting. I think that is what I would like to experiment with next... some crazy hues of colour, within the correct shade of light and dark. For now I am just attempting to finish this one first!
