I have never done it, but here is a good website that describes the process.

Here is a link that might be useful: Grout colorant restore

Hello Graywings,

Thank you so much for the link. Will definitely look into the products.

From the article, it seems now is a good time to apply the "paint" since tiles and grout lines have just been deep-cleaned.

I used the Custom brand grout paint at Home Depot. I believe it is an epoxy based sealer with color....so a paint. I used a used child sized toothbrush and it was time consuming but easy...and very rewarding as the white grout was very stained. i used white because the tile was white and the tile install was just okay because it was done by a previous owner---accent color would make the layout stand out. Grout almost always looks better in exactly the same color as the tile. I cleaned the excess off as I went. It was in a rent house but I am confident that the product will perform...custom is an excellent company and epoxy is an amazing substance. I believe I was very pleased with the aesthetics too, the grout looked like sanded grout that was a little shiny as expected of sealed grout. not painted. I was however very careful not to over apply. I don't know how to add a photo.

Hello Marian,

Thank you for the photo, the grout looks very nice, this is very encouraging.

Now it has been a few days, does the grout still look as good as when you first painted? Could you please post more photos?

Here is a link about adding photos:

http://ths.gardenweb.com/forums/load/kitchbath/msg0213484417254.html

When we put our old house up for sale, I cleaned and painted the grout lines in the bathrooms. It sure made a difference. I think I got the product from the local hardware.