I tried the Commercial grout cleaners and wasn't too happy with the brand that I purchased. I have light tiles and light grout (when clean), so here is what I used. Clorox Clean Up worked as well as Scrub Free Mildew remover. Let it sit for 15 minutes and scrub. For the toughest stains (now don't laugh) I ground up denture cleaner tablets and placed the powder in the grout lines and added water. I also let this sit for 15 minutes before scrubbing with a brush.
Don't let the color of the tablets scare you, it does not stain. This worked for me and we are back to the original color.