For a natural yet powerful solution, baking soda and vinegar are your go-to combo. Start by sprinkling baking soda directly onto the grout lines. Then, pour vinegar over the baking soda; you'll see it fizz as it gets to work.
Let the mixture sit for about 10 minutes before scrubbing with a brush or old toothbrush. Rinse with warm water, and your grout will look noticeably brighter. This method is ideal for light to moderate stains.