For our downstairs bathroom, we used a grout pen to cover up the dirty grout. Although it's chipped a bit within a year or so, it was a decent solution for that space. But our upstairs bathroom was super gross. It was really discolored and I felt like the pen wouldn't really cut it. I went to Home Depot and found this paint made for grout. It sat around for a few months and after seeing the same exact paint over at yhl, I was motivated to get off my arse.
There are other options that I wish I had considered more heavily. You can actually re-grout over your existing grout [or so I've been told] assuming that your original grout is in good condition and not cracked. Or dig out all of your grout and then re-grout. In all truthfulness, I really wanted new tile for that bathroom. I've loathed this tile since we bought the house but our realtor didn't think it would be something we would get our money back on so soon. If we were living here longer, I'd consider it. So I sprang for a $12 bottle and set out with a bulk box of Q-tips. Here's what we started with:
Ugh. I guess it's just years of living but it always gave me the heebie jeebies. I made an awful mistake as well and started off with a gray grout. I didn't listen to that little designer gut feeling I had. In the end it made the white tile pop, which would've been fine if I liked the tile in the slightest. But I don't. So after doing 3/4 of the job, I switched to white and started over again. It took several coats to cover it up.
At this point I was feeling really frustrated. Even with a Q-tip it was really difficult to stay in the lines. I was using a ton of toilet paper to wipe the edges and it was taking forever. I ended up throwing caution to the wind and coloring outside the lines without abandon. I went back later and used a glass scraper to clean up the edges. Obviously if you had a tile that was textured, this wouldn't be an option. And I pity you.
So I can't really say I'm happy. It was more than difficult to apply [or maybe it's just my lack of fine motor skills?]. It needed multiple coats, even in the areas where I didn't go with the gray ahead of time. It looks built up and ugly in certain areas. But I will admit that unless you're really on your knees looking at it, it's not that bad. Standing at five feet up, it looks okay. But just okay. I'd only consider this as a band-aid solution. Something to ease your mind that grout germs aren't going to attack you until you can afford a more permanent solution like a complete re-do. So I hope that helps some of you if you weren't sure what to do!