Value is the brightness of a color. Dark green and light green have different values for example. Yellow has a lighter value than blue, which is quite heavy. The change in value across the surface of the image helps to create silhouettes, planes and depth- the illusion that the image made on a two dimensional plane exists three dimensionally.

Since values are commonly used to describe shapes and create contrast, it’s important to not ‘waste’ highly contrasting values on too many unnecessary details. When you flood a painting with contrasting values, it becomes tough or even impossible to pick and bring important features and elements to the forefront of the image, making it chaotic and generally difficult to read through the details. On the contrary, it can also be too easy to be too subtle and low contrast with values, ending up with a muddy image that generally lacks discernible features.

Although you can get an idea of what values are used by converting a color image to gray scale, there’s a bit more to it than that. The human eye can be tricked in many ways. Bright yellow can sometimes seem brighter than white, and dark purple can seem darker than black. A dark grey dot in a large white field will seem darker than it is, and a light grey dot on a black background will seem white. Both colors and values are affected by their neighbors on the canvas.

If your painting isn’t working no matter how much you tweak the colors, it’s probably the values that are wrong, not the hue or saturation of the colors.