Sometimes using a piece of red gel (acetate)- as in lighting gel – will help you see the values of your subject and painting better. This seems to work best with landscapes as the red gel counteracts the greens, just leaving the values.
I’m not quite sure where I came across this idea but I always carry a piece of red gel with me. You can acquire leftover pieces from most theaters in your area, even some universities. They use the gels to color the lights for the stage. Or you can buy it new on Amazon.com or at a theatrical supply store. If you use a viewfinder, there is even a really neat gismo which you can buy at www.pictureperfectviewfinder.com which has the red gel built in, along with some value markers and different size openings for standard painting sizes. Try it. You’ll like it.