Light and dark in a painting can refer to more than one thing:

2. Pictorial Light
In representational paintings light and dark can refer to the illusion of different kinds of physical light, such as (a) the radiant light of the sun or a candle and (b) the light reflected from surfaces that makes objects visible. This illusion of light is called pictorial light.

In this painting the 17th-century French artist La Tour has used a range of light and dark pigments to create the illusion of the radiant light of a candle and the light reflected from the infant Christ and the shepherds. Notice how La Tour used the lightest pigments to paint the infant and how this dramatic use of light focuses your attention on the small child.