How can I clean the grout lines on my bathroom floor?

I use bleach water. Followed by baking soda water, then clear water. After dry, put down grout sealer.

hydrogen peroxide and a old toothbrush, let it sit for about 15-20 minutes

There is a toilet bowl cleaner that is a bleach gel, you run some down the line, let it sit just a few then take a wet brush and scrub. Comes right up. A bit of a pain< had to do my kitchen that way, but it works!

paint all of the grout in a dark color such as black

i agree with using any formula including bleach and letting it sit a bit- - there are specific grout brushes available at the hardware store. The job is labor intensive, but it works. Finish by applying a grout sealant and you can avoid the deep leaning in the future

wear goggles, the dark grout lines won't be near as bad as bleach splashing in your eyes..