How do you clean floor tile grouy?

high traffic area - hallway, kitchen, eating area and front entry - have grout that is now very dark brown/grey. It wasn't that way previously. Its had about 6 years of traffic and wasn't sealed (to my knowledge - tract home - I was original buyer) If not able to clean, can I re-color it or would I have to re-grout the entire area, a proposition I do not look forward to.