An artist will use a shape unlike the shape next to it, or a color opposite of the color next to it, in order to show contrast. Contrast can create interest in a painting by making certain parts of a painting stand out from other parts.
In the Louisa Villea, contrast is used in several ways. Louisa's red gown contrasts with the green background. The realistic highly defined portrait of Louisa contrasts with the loosely defined background buildings and the swirling, indistinct abstract shapes.