They allow less heat to escape. This spring, inspect and clean both the indoor and outdoor coils of your air conditioner. Dirt build up on your coils is the single most common cause of poor efficiency. Insulate attic access and basement trap doors with R-19 insulation. Paint and decorate in light colors.
Dark colors absorb light.
Light colors reflect light. The lighter the colors you use, the less artificial lighting is required to illuminate the area. Check your lamp shades. Many shades are decorative but absorb light or reflect it in the wrong direction. Without us attempting to be your interior decorator, you may consider switching to a more efficient shade in a lighter color.