How to Judge Values in Oil Painting.  By Don Finkeldei:   One of the hardest things for an artist to do is develop a natural  ability to judge values in a  plein air landscape scene and then translate that value to the canvas.

There's a huge difference between the value range that nature uses and the value range you can use on a canvas.  The reason is: 

The light In nature (and tv screens and computer monitors) is additive.   it changes color and brightness by adding light of various wavelengths to it. All color is "ADDED", making the value range wide and intensity greater than you can do with paints on a canvas.

A canvas doesn't do that.  It REFLECTS light  by a subtractive process.  It absorbs various wavelengths of light  and only reflects the colors that haven't been absorbed.  Your eyes see only the diminished intensity of the colors that aren't absorbed.  Most of the brightness is greatly decreased. 

You might ask why a mirror reflects everything back to you in the same intensity.  A mirror is not composed of opaque pigments that absorb.  It's a true reflector.  It bounces back ALL wavelengths of light.   Paints can't do that and never will.  Nor do you want pigment paint to do that.  You're trying to capture a scene... not reflect  an image.

The value range in nature, TV monitors, and computer screens are additive.  They  can have a value range (and intensity) that is hundreds of times greater than the reflective value range you'll use  on your canvas.

So what can you do?  Well, one thing you cannot do is put some paint on a palette knife, hold it up to the scene and try to match the value in nature to the value on your palette knife.  (nature is additive, the paint on the palette knife is subtractive).   What you can do is mentally set up equal value scales for nature and your painting.  Like a range from 0 to 10 (10 being the darkest).

Try to find the lightest and darkest value areas in the natural scene.  Mentally assign 0 to the lightest and 10 to the darkest. 

Mix some paint for  on your palette defining the lightest and darkest value areas.  assign 0 to the highest value and 10 to the lowest value you mixed.  You've just  established equal value scales.  Nothing in your painting can be lighter or darker than the values that  you just  mixed.

As you are painting, try to think in terms of  values related to these value scales.   Also, judge the values of the surrounding objects/areas.   For instance, a foreground field might have an overall value of 3 in nature.  When you mix paints for that area on your palette, you know that you have to mix it at a value of 3 compared to the 0 value and 10 value you mixed previously on your palette (the nature value relative to your palette value..

Color will fool your eyes.  Yellow is in the middle of your eye's visible spectrum and influences the intensity that your eye sees.  Hence, your eye is more receptive to yellow than it is to blue or red.  It appears as a higher value (and more intense)  than it really is.  You have to think in terms of gray.  A good exercise is to create a value scale on your palette using black and white paint (Or very dark gray and white, which I prefer.  A dark gray mixed from the primary colors on my palette).  Then,  put a dab of cadmium  yellow below it.  Take a digital photo of your palette in gray scale.  You'll see what the grayscale value of your yellow really is, It'll surprise you how much the yellow influenced your value estimation.

I usually mix pools of color/value that I'll use in a painting,  I arrange them  in areas on my palette that correspond  to the most common value planes in a painting.  See Types of Light and Value Planes.