To freshen up your carpet's color, sprinkle a light layer of salt on your carpet. The salt acts an abrasive. As people walk over your carpet it will sharpen up the color.

If you have moved furniture and the nap of your carpet is matted down in places, place ice cubes on the matted down areas overnight. The nap of your carpet should be back to normal by morning.
Source: http://www.cleaningtips.com/tips/house/carpet-cleaning/freshen-carpet-color-matting.html