Evaluating Values in Photoshop.  By Don Finkeldei:  How to evaluate values in your painting by using Photoshop to turn a color image of your painting to grayscale.  Doing this will greatly accelerate your learning curve

 As you know, color can fool your eye when trying to evaluate values.

Take a digital picture of your painting.  Open it in Photoshop.  If you need help with imaging software, read the other articles.  Once you have the image open in Photoshop, go to Image>Duplicate.  That will make another copy of the image so you can turn it to grayscale and still keep your color image.

To turn the copy of the color image to gray scale, Go to Image > Mode > GrayScale.  That will remove all color info on the image.  Now you are looking at only black and white values.

It’s much easier to judge the values looking at a grayscale image as you can see.  Notice that values in the shadows of the mesa are much closer in value than you’d guess just by looking at the color image.  Changing color and holding the value the same is a very desirable quality in a painting --- especially in shadows.