Back when most walls were plaster, paint with a flat or matte finish was the obvious choice because it hid imperfections. Paints with more sheen—eggshell and satin—were a good bet for the bath or kitchen because they cleaned up easily. That left semigloss and high-gloss finishes with the job of protecting wainscoting, doors, and trim from dirt and hard knocks. Today's finishes, however, are better performers all around, so there's no reason not to experiment. If you're painting foyer walls an intense color, like denim blue, consider intensifying the effect with a glossy finish. If you're softening up the living room, try unifying walls and trim by giving them the same medium sheen. As for flat, it's not over yet: It's still a great choice for walls because it has a light-absorbing quality that enhances deep, rich colors.