its quite simple - just have the ability to assign a color that is other than black to the vignette.
Here are some examples of different colored vignettes:
here is a red colored vignette in a game - designed to show damage:
http://www.mindorchard.com/images/conte ... ofduty.jpg
http://s28.postimg.org/cw0xjh7rx/vignette2.png
blue vignette for atmosphere(half life):
http://files.gamebanana.com/img/ss/guis ... baac23.jpg
white: http://i.funny.pho.to/preview/vignettin ... ffect.jpeg
blue: http://cdn.digital-photo-secrets.com/im ... gnette.jpg
http://cache3.asset-cache.net/xc/499130 ... ATgORYAR20
pink: http://previews.123rf.com/images/wavebr ... -Photo.jpg
Just google image search 'vignette+color' - you will get a billion results <img src="{SMILIES_PATH}/icon_e_smile.gif" alt=":)" title="Smile">