Has your grout shown it's age and in need of some attention? If your grout is in good condition meaning that it's not crumbling or falling out of the joint, then a quick touch up may just what is needed. Make sure and read how to clean grout before attempting to color match your grout. The new grout will stick much better and last longer too.
If your grout is beginning to crack or crumble, then it is generally a good idea to remove all loose grout with a utility knife before applying new grout. If your grout has been attacked by mildew or mold, you may try cleaning your grout really well before deciding on whether to replace it.
Maybe you have been considering changing grout colors while keeping your original tile that is in great condition. This would be a perfect opportunity change colors using a grout colorant! Just a little cleaning and grout touch up will prepare you for a professional grout color change that will make your tile look new again!