When we moved into our former home 28 years ago, the shower was a dull blue because of the soap scum and the shower pan was gray with scum, too. I tried everything available on the market. Nothing worked.
Finally, I took a single edge razor blade and 'shaved' the scum off. The tile was actually a very pretty blue, and in excellent condition. But it took HOURS to do. I didn't want anyone to take a shower after that!
After that, and to this day, the last one in the shower has to squeegee it down or wipe it down with a towel. Cuts down on mildew, too.