I wouldn't use bleach, as it will actually break down the grout... I really am not sure what to use on them that would work best, though.

pure sodium percarbonate (oxygen bleach) - it works wonders - I use it on all sorts of things

Watch this link. You can buy pure sodium percarbonate from several sources (Ecover sells one).

http://shop.askthebuilder.com/categories/Stain-Solver/

I'd try pouring or wiping or dabbing peroxide on the grout. Peroxide is basically an oxygen cleaner, not sure if it will react chemically and break down the grout. Does anyone know? When we re-did our bathroom floor with tile, we chose a gray grout. Looks dingy all the time but at least we chose it and it's not really "dirty". By the way, I clean my tile and grout once a week with Cinch spray.

I was told to use pinesol has a little oil in to keep from drying out alsoabout once a month take rag put mineral oil on grout to protect it.

I use the Shark handheld steamer. It works great on the grout of my marble tiled shower without injury to the marble.

I'm thinking hard about getting a steam cleaner. So many uses!

Mr. Clean Magic Eraser. I was an idiot and laid a kitchen floor with really wide white grout lines. Thankfully I spent the time and $ to seal them but they still get dingy. The eraser brings them right back to white with just a few swipes.