Why can it be difficult to color match grout?

Grout will change shade over time as it oxidizes and also as it absorbs dirt and grime. Its color can also change as it gets cleaned over time. So even if you know the exact brand and color and type of grout that was used previously, it can sometimes be difficult to get it to match if you're trying to do a patch in an existing tile that was done more than just a year or two previous. Though it can be difficult, there are a couple tricks that you can use to dirty up grout a little bit or brighten it up to ease the transition from one slight shade to another.