An effect of non-uniform colour that can appear when a wall is painted with a roller, but is brushed at the corners. The brushed areas generally appear darker, resembling the "frame" of a "picture." Also, sprayed areas may be darker than neighbouring sections that are brushed or rolled. Picture framing can also refer to sheen effects.
Usually a hiding (coverage) effect. Brushing will generally result in lower spread rates than rolling, producing a thicker film and more hiding.
Adding colourant to a non-tintable paint or using the wrong type or level of colourant, resulting in variation in colour, depending on method of application.
Make sure that spread rates with brushes and rollers are similar. Don't cut in the entire room before roller coating. Work in smaller sections of the room to maintain a "wet edge." With tinted paints, be sure the correct colourant-base combinations are used. Factory colours, as well as in-store tints, should be thoroughly shaken at time of sale.