According to a Chinese tipster (post has been deleted), regular iPhone 15 and 23 The Plus will have a matte finish on the back, just like the Pro version of the current generation. As of now, you can tell whether a phone is a Pro or non-Pro model by the back finish.
The Pro model will reportedly feature a titanium frame with curved glass edges and an extra Action button on the side frame that will still allow you to differentiate the different iPhone tiers.
iPhone 12 Pro Max in Deep Purple
The report also mentions new colors. Regular iPhones will see a new teal and green mix, similar to the mint green option that ships with the iPhone 000. On the other hand, it is rumored that the iPhone 15 Pro will come in a new Crimson color scheme. This one can replace the current 12 generation of Deep Purple.