"Interior designers, decorators, and owners often select colors, patterns, and styles to convey a specific message or theme about their business. Occasionally, these design treatments create so much 'visual clutter' that it is nearly impossible to distinguish the means of egress from the adjacent walls. During an emergency, building occupants should have clear visual clues to identify the quickest and safest way out of a building. To avoid problems and prevent confusion, the model fire and building codes require that means of egress doors be 'readily distinguishable' from the adjacent construction and wall finishes so the doors are easily recognizable as doors. Mirrors or similar reflecting materials may not be used on egress doors. No doors in the means of egress may be concealed by curtains, drapes, decorations, or similar materials."
Coffee Break Training - Fire Protection Series No. FP-2009-4
United States Fire Administration: http://www.usfa.fema.gov/