The easiest way to clean grout is with a steamer as mentioned. Just use it the same way you would to steam clean a tile floor. If the steamer doesn't do the job by itself then you most likely have stains in the grout. Visit your local cleaning supply company and explain the situation to them. They should have a product on hand that can be used to clean grout. If that type of cleaning doesn't work then the only option is to remove the grout and re-grout again. In any case, whether new grout or newly cleaned grout, you should seal the grout once a year to keep the dirt and food off the grout. This will keep it from staining again. Once again your local cleaning supply company will have what you need on hand. This once a year sealing step will preserve your grout like new for many years. Grout though very hard to withstand walking on it is still a porous material that will absorb food and dirt. When this happens the grout can be discolored beyond repair. Hope this helps.

I'm not so sure anymore. I recaulk for lots of folks and see the same mold issues everywhere, even with folks that clean frequently. And I also religiously follow the mfr. directions, still to little long-term avail. Yes, I know caulk is flexible and moves with the joint to maintain the seal, but the bad part about that (for the horizontal joint, especially) is the moisture that gets behind is "sealed in" and doesn't evaporate well enough, hence the mold, etc. And that's even when the joint is properly cleaned and dried before caulking. Note that a certain amount of moisture penetrates the grout higher up and travels downward in the wall. That's why there's a lip on the tub onto which the tile and backerboard overlap. Sometimes I'm inclined to find a way to grout instead, and then repair when necessary. Perhaps if a rigid, thin piece of plastic were temporary installed, then when the grout dried, one could remove it and have a "hardly-noticable gap" for water to drain and/or evaporate. They need to come up with a permeable caulk for that horizontal joint.

Well, to be optimistic, it's working for what you need right now...I do have the feeling that the toilet clearance is not to code tho... My guess is that the current tub/shower combo has some kind of damage to the outer wall (not the part that holds the water) and the previous owners got it cheaply and covered it up. In your position, my future plans would be to demo just the front. The amount of damage that was existing or I created with the demo would be the deciding factor of what to do with it. Some cosmetic damage can be creatively covered (NOT like what they did...). If that's not possible, I would replace the whole thing. Basically one step at a time. Right now, though I would remove the grout. You're the only one who knows exactly how much gets wet (whether the wall grout or just the deck grout). If it's just the deck, I would try to crack that section in the corner so I can remove the rest...maybe a flathead screwdriver and a gentle tap with a hammer? Then caulk.

Fwiw, I think it looks good as is and wouldn't bother recolouring the grout for a few years still. Your tiled walls read as white to me. The grout pattern is subtle. When I told the tile showroom consultant I wanted white shower tiles, she advised me to go with an off-white grout, not bright white. The tile's "shadow" colour, in her words, which is often a very light grey. I used a grout pen in my old bathrooms to brighten up the old beige grout. Tedious but rewarding job. So far so good on shower walls (over a year). Recently did the floors - don't know how long that will last.