Probably not durable. If you want to maintain a perfect finish, you should use a suitable case to protect your device. If you don’t use a case, you’re only one accident away from a dent, a scratch or a paint chip.
I’m hoping the rumored brushed finish will help hide potential scratches. As far as the durability, who knows? I think the finish on the 14 pros were pretty sturdy but other people have said otherwise so 🤷♂️.
My Ultra have a few scratches that hard to see unless you put the Watch up to your eyes. I hope it’s as durable on the 15PM, although it won’t really make a difference to me since I’ll be using a case. I’m going to miss the stainless steel sides on my 14PM, which also barely have any scratches.
My old Apple Watch s7 edition was space black titanium. If they’ll use the same/similar finish for the ip15 pro line, I’d say it’ll be pretty durable. I never went ham on it btw, but daily knocks and bumps were no problem.
So as of now, there is no Ultra phone, nor is there anyone here with a titanium phone with a color. So your question can't be answered. People can speculate but that's all it will ever be until the event on the 12.
Since we will not know until the product is out in the wild but I can reasonable expect this forum will be full of threads of terrible that the finish does not stand up.