Value

In another section of this site we talked about Contrast. In some instances, value is how we get that contrast. "Value is a measurement of the brightness of a colour. The brighter a colour is, the higher is its value and the more light it emits. For instance, a vivid yellow is brighter than dark blue, therefore its value is higher than that of the blue. A good way to see the difference in the values of colours is to look at the corresponding greyscale version."

So basically Value aids in contrast.