An appearance issue where the brushed areas of a paint coating generally appear darker, resembling the 'frame' of a 'picture.' Sprayed areas are also often darker than neighbouring sections that are brushed or rolled.
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
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, and the product must be thoroughly stirred prior to use.