The value of a color or an object depends on what is around it. If a light gray object is surrounded by white it will appear darker than if the same gray is shown against a black or very dark background.
You can try this yourself. Divide a paper in two, half light, half dark. Cut a small square of gray in half and place one piece on each of the two backgrounds. Notice that the gray chip on the darker color appears lighter than does its twin on the lighter background. This is called the law of simultaneous contrast.
I tend to paint too lightly, not getting my darker values in place from the beginning. So I play a trick on myself by taping my painting surface to a black board, using black tape. I got the basic idea for this from Master Pastelist Richard McKinley. Doing this encourages me to go darker right at the start than I would otherwise. Then when I take my finished work off that board, voila! It has the dark values as dark as they need to be.
The law of simultaneous contrast shows how important it is to understand something in context. Life is a bit like that don’t you think? If we don’t know the context to a remark or a look, we might misinterpret it as more complimentary or more insulting than it is intended. Context is everything!