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

1. Lightness or Darkness of Pigments
Light and dark can describe the pigments used in a painting, such as light or dark blue, light or dark red. The terms also describe the appearance of areas of a painting created with light or dark pigments.

This fresco (wall painting on plaster) from the ancient Minoan civilization of around 1450 B.C.E. is painted with different colors (cream, blue, brown) that range from light to medium to dark. The light colors of the figures on the left and right contrasts with the dark browns of the bull and middle figures.