Why should painters judge value before color?
Short answer: Value, how light or dark something is, carries the form, the light and the likeness in a painting. If the values are right, a wide range of colors will work; if the values are wrong, no color choice can save the picture.
What the eye reads first
From across a room, a painting is read as a pattern of lights and darks. Color is noticed second. That is why a black and white photograph of a good painting still works.
How to see value
Squint until the detail disappears and only big light and dark shapes remain. Compare two areas directly: which is darker? Photograph your painting and turn it greyscale on your phone to check yourself.
Exercises
Paint a five step value scale, then match five objects on your table to it. Paint a simple still life in black and white only. Then repeat the same setup in color while holding the same values.