Hi Nancy, You can bleach it or paint it. I have given up doing either and decided to darken all the grout in my floor using liquid shoe polish, which does it for me. Best wishes....
They sell grout paint pens!
Use Clorox Toilet Bowl Cleaner which works very well for me. It is a gel so put a bit in the corners of four tile, spread through the grout lines with a toothbrush, let sit 10-15 minutes and then scrub again with the brush. Clean the soap residue off with warm water and some vinegar. When you have done the whole floor then damp mop with the warm water and vinegar mix to be sure you have cleaned it well.
here is my all time favorite grout cleaner that works great! http://gatheredinthekitchen.com/2013/01/09/diy-grout-cleaner-homemade-recipe-with-baking-soda/
Sprinkle comet powder followed by hydrogen peroxide then scrub all over especially the grout (it’s mostly dust buildup that is ground in that makes it darkened) let it sit 15 minutes and rinse off it may take a second time to get perfect results- I have heard vacuuming tiles every couple weeks will help avoid the build up- probably a sealant would be good after whitening-
You can trace the grout with bleach on a Q-Tip. Let dry (don't breathe the fumes!)