Even if you vacuum weekly and professionally clean your carpet yearly, it gets the uglies. A lot of times because of wear patterns and matting, however a lot of times it is from fading.
Carpet gradually looses color and it usually doesn't get lighter of the same color. No the primary colors separate and the carpet can get a green or orange tone. What's up with that right?!?
Direct sunlight of course, but also Ozone fading. Ozone fade is pollutants in the air mixed with humidity and when it settles on carpet acts like a peroxide and causes fading.
Bleach spots, Kool-Aid stains, pet accidents and "traffic lane" grey are a few unsightly things that go away when the carpet is dyed completely.
Say What?!? Lol Seriously I fixed a carpet that a decorator had just put in that didn't match the sample she had to coordinate with the drapes. I sprayed some gold dye on it and saved the day, otherwise they would have had to replace the carpet.
Wall to wall dyeing can extend the service life, of carpet, to aid landlords in getting another tenant before it has to be replaced.
It won't be new carpet but some say it looks like new when I'm done. Dyeing will not improve the worn traffic lanes. So if it's really worn, save your money and buy new carpet.
Dyeing also won't help your soiling issues. If you think dyeing the whole carpet will keep a carpet from getting dirty so soon after cleaning... it will not, but it may help to camouflage the soil. I usually recommend that you just clean the carpet more frequently. It will be less costly to do that too.
However dyeing a carpet darker may be easier to maintain.
Carpet Color Change Part 1
Carpet Color Change Part 2
Carpet Color Change Part 3