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 the picture. Also, sprayed areas may be darker than neighboring sections that are brushed or rolled.
Make sure that the spread rates with brushes and rollers are similar. Don't cut in the entire room before rolling, work in smaller areas and maintain a 'wet edge'. With tinted paints, ensure the the correct colorants and combinations are used. Factory tinted colours and colours tinted in-store should be thoroughly shaken at the time of sale.
Source: The Paint Quality Institute