Seriously wish I found this sooner!

Make a paste with lemon juice and baking soda. Apply to the stained grout and let it sit for around 10 minutes. Scrub with a brush and rinse off with warm water. 

Add a few drops of tea tree or eucalyptus oil to a cup of water. Spray onto the grout, wait a few minutes, then scrub and rinse. The antibacterial properties will help clean and freshen the grout. 

Mix salt and vinegar into a gritty paste. Apply to grout lines and scrub. Allow it to sit for a bit before rinsing with warm water. 

Dilute liquid castile soap in warm water. Scrub the grout lines with this mixture. Rinse off with plain water for a clean finish.

Combine borax and lemon juice into a cleaning paste. Apply to the grout, scrub with a brush, and rinse off. The acidic nature of the lemon paired with the cleaning power of borax works wonders.