Here is a room with some of the colors you mentioned, for an idea. If you do a turquoise wall, I would try to tie that into my rug somehow/or some of the accessories

yes to all, we did have the tiles professionally sealed, but sealing is not a cure all in my experience but I'm sure it helps considerable . my wife does buy those hand made goats milk styles soaps etc and I don't imagine there is anything more testing for stone, and yes it does mark. The Grey ( bane of my existence )granite much more so than the beige travertine ) I must admit though, the kids bathroom was almost non existent as one has sensitive skin and uses pump pack. I would also imagine climate, airing/ sunlight, volume of use and the time you have to manage it would also be contributing factors. We are way short on time and they don't get the attention they deserve, so I would suggest asking around, others may manage it better and it not be an issue. If your only doing your bathroom you may consider the risk factor not worth it for the volume, but for me, the advantage of travertine in the rest of the house far out weighs any marking issue in the showers, and wont stop me from using it again. To be honest when we sold the last house I just took to the shower with one of those paint scrapers with the razor in the end and most of it came off and it didn't detract from the look of the house. Sorry i have probably only muddied the waters even more, good luck with it.

Maybe check out Smartstone Athena. I think it might fit your criteria.

Hi! As a Stonemasonry and Glass company we come across the dilemmas of splash back colours continually. I do recommend if you were to choose the white to get it colour matched to the white background of the stone. This option will give the kitchen a "less is more look" and your eyes will e drawn more to the stone as the feature in the kitchen- The mirrored splash back finish will open up the room and make it feel more spacious due to the reflective surface- so is great for a compact space or smaller room.