Haloed edges can be used to symbolically express an emotional or psychological characteristic of the subject. The halos might be stronger where they meet the edge of subject/shape, growing more diffused as they get further away from the edge. This would give the sense of an emanating aura. Or, the halos might be a solid band or shape encircling the subject, in which case the halos might take on a more graphic/abstract quality. The use of different colors and/or values in the halos around each of the subjects/shapes in a piece might be used symbolically to communicate a characteristic of each subject/shape.
Featured: Worker Boy by Egon Schiele.