Grout Renew

if its in the bathroom, you can buy a white paint pen that has in it to cover the earth ...

if its gray, then just wipe with a dry cloth and it should revert to the original color / color.

Zap has a product for restoring grout. It works very well. You can buy it at Home Depot. You just spray it. Zap-Restorer.com When you're finished I seal the grout. It makes it cleaner for longer.

None of these answers. Go to Home Depot or Lowe's and buy a product called Grout Colorant & Sealer by a company called Versa Tile. Buy blank if you want a pure white. It's like a tube with a small brush short on hard top. Shake well and open the lid and the product will be available on short brush that you apply to your grout.
Easy and it makes your grout Search BRAND NEW. I did my kitchen and bathroom above all, we sold our house. My neighbor paid a guy to regrout and I used this about 3 (a tube of dollars) and mine looked better than hers! A tube does an entire room so you do not even need much.
Good luck.

Try toothpaste and a toothbrush.

Either make a paste of scouring powder and Ajax bleach put in the grout lines to keep a certain time, and remove. However, there is a product for the renovation of grout tile section ... Homebase

How about using the Clorox and let it rest for a while (if the soil). If it's on the walls, soak and rinse.

I recently saw grout pens "for sale, still think I'll try it myself.

Lightly rake.

You should be able to buy pen injection of any hardware store. It is very easy to use, just mark it on the grout
as if you were to color. I used one and get very good results, especially in the bathroom.

Scrape off the surface.

Mix lemon juice and bicarb soda together, then rub in leave it for one hour or two, then wash as normal.

You can buy grout clean. I use a solution of the ladder in the water, it brings back looking like new

painting

use bleach and a toothbrush

http://www.screwfix.com/app/sfd/cat/cat. ...

Get a Mr. Clean magic eraser. You can find in any store like Kroger or Wal-Mart and others. Am sure you've seen on television in commercials.

Use full strength white distilled vinegar - spray and let stand a few minutes - scrub with old toothbrush - and rinse with water.

Toothpaste

Use Greased Lightning ...... then seal the grout to prevent new spots