A non-uniform color effect that can appear when the corners are first cut in with a brush, and then the walls are rolled. The brushed areas generally appear darker, resembling the “frame” of a picture. Also, sprayed areas may be darker than adjacent sections that are brushed or rolled.
Usually a coverage effect (brushing will generally result in a thicker film than rolling)
Adding colorant to a non-tintable paint or using the wrong type or level of colorant
Make sure that the paint is applied evenly when brushing and rolling.
With tinted paints, be sure that the correct paint base-colorant combinations are used.