For tile walls, try using a bleach pen to transform your grout from grungy to great. This method is tedious, but the payoff is crisp, clean grout lines. Use the pen to “draw” bleach across the grout lines. The pen allows you to target the grout without getting bleach all over the tile. Wait 10 minutes and then rinse.
For really mildewed grout, you may need a second application, and it can help to gently scrub the bleach into the grout with a toothbrush before allowing it to work for 10 minutes. Make sure to run the fan in the bathroom and to avoid skin contact. This method is best for light or white grout. If you have coloured grout, test a small area first. It might fade.