Which Square is darker, A or B? And yes, it's a trick question. Color and Value are simply an impression made on the optic nerve attached to the brain. That impression or perception is altered depending on the relationship of the color and value to other colors and values around it.
Nothing has illustrated this point more to me than the Edward H. Adelson photo of a green cylinder casting a shadow on a checkerboard, all in gray-scale.
As artists, we know shape and value are the most important elements to convey accurately on our paintings. But is it Value, how light or how dark to mix our color, that is the most troublesome.
Green is not simply green. The acid spring green of trees as they begin to leaf out can be more akin to yellow than to green, and the green of a Magnolia leaf in shadow is closer to black than to green. Both are green, yet one resides on the high end of the value spectrum and the other on the low end.
Practice looking at Values this week, and the relationship between them. Work that muscle, and it will pay you back in dividends for years to come in your paintings.
And if you ever feel something is "off" in your painting, you can bet it's either shape or value, and more likely. it's value.