Interior trim can give a room balance, definition and proportion; highlight points of architectural interest and ornamentation; and set its tone and character. It can take the form of window and door casings; baseboards; wainscoting and paneling; pillars and mantels; and crown, picture and chair-rail molding. Think of trim like the frame of a painting: It outlines and enhances the beauty of what's inside.
At its most elemental, trim adds punctuation to a room's elements. Crown molding signals that the wall is ending and the ceiling is beginning. Baseboard announces the transitions between wall and floor. Casings define the perimeter of doors and windows.
But trim also has the aesthetic function of enhancing or changing the perception of a room's size and shape. By using trim to create strong lines, you can make a room seem larger, or by breaking up soaring walls, you can make it seem cozier. Through the use of trim you can both enhance and complete the design style you envision for the space.
Both the architectural style of the home and your client's personal taste will play a large part in both the style and color options for trim. For example, heavy Victorian trim does not suit a ranch-style structure or an Arts and Crafts cottage, just as minimalist trim lacks the impact required for a Victorian or Georgian design.
As with any design decision, when selecting the trim types and colors for a room, you have to determine the end goal. What impression does your client want the room to convey? Relaxed or stimulating? Modern or classical? Sophisticated or eclectic? Dramatic or formal?