Though it dries to a cement-like finish, grout is actually a soft and porous material. It absorbs stains easily, especially if it is not sealed. There are several commercial grout cleaning products on the market; however, many of them include harsh cleansers such as bleach that can fade or change the colour of your grout. It is possible to remove yellow stains from grout without harsh cleansers, but it is easier to prevent stains by sealing properly and cleaning regularly.
Mix powdered oxygen bleach with warm water according to the manufacturers directions. Many companies make an oxygen bleach product. Writing for the website Ask the Builder, home renovation expert Tim Carter suggests purchasing an oxygen bleach product that contains the highest amount of active ingredient and does not contain a high number of additional ingredients.
Apply the oxygen bleach solution to the grout with a sponge. Allow it to soak for 30 to 60 minutes, depending on the severity of your stains.
Scrub the grout with a soft-bristled brush.
Mop or wipe the surface with clean water and allow it to dry. Repeat the process as needed until the yellow stains are completely removed.
If your grout stains easily, it may need to be resealed. Do not apply sealant over stained grout. Remove grout if it cannot be completely cleaned and regrout your tile surface. Help avoid set-in stains om grout by cleaning up spills as soon as possible. Use a paste of white vinegar and baking soda for regular cleaning.