We've been in our house for 2 years (bought it new) and a few months ago, I could hardly stand to look at the grout. It was discolored and gross no matter how much you cleaned it. I was having dreams of having to have it all replaced...it was that bad. A company who "restores" the grout wanted to charge me $2000!
I bought PolyBlend Grout Renew paint from Home Depot for $12 and the results were amazing! It was a little time-comsuming to apply - I first cleaned the ALL the grout by hand with a firm grout brush and then used a toothbrush to apply the paint. If you use a wet towel as you go, it's pretty easy to wipe up the extra that gets on the tile. I have probably 2000sq ft of tile, but I did a little every day and it took me about a week and I only used one bottle.
The first bottle I bought was too light (looked like white-out), but once I got the color matched right, it looked great.
Needless to say, I highly recommend it!