It is. What is happening here is that the underlying substrate is a metallic media. Hence, the white coating will yield to the media.
It happens in print media as well.
The whole point of certain coatings (beyond protection or feel, so in the context of "color") is to alter the light reflected or absorbed by a material. Your assertion that because, somehow, there maybe some Titanium Dioxide (which would imply that it's been sufficiently exposed to O2 and allowed to, you know, oxidize) somehow "overrides" or "overpowers" or whatever you'd characterize it
from underneath a coating designed and engineered precisely to permit or not allow such wavelengths to absorb/reflect as the coating on top is exactly the opposite of the billions of dollars a year coatings industry that has successfully (for decades) produced coatings that alter the appearance of titanium (and additionally, slow its oxidation to a near standstill over the life of the product).
There is no stronger argument here than reality: they are selling black and blue iPhones, so, clearly, there is a process and a coating (or set of coatings) being used to mask whatever amount of oxidized titanium maybe there as a substrate.
That is, after all, what the prefix "sub" means: under.
Glad you bring up print media, actually! How, in the world, then is not everything printed completely white when printed on white paper (which was generally bleached to get it there, anyway)?
Because, well, ink/dye/pigments are a thing. Like you said, it's no different.
Over time, through tons of wear, perhaps all coatings will wear off, then the titanium will oxidize over time and create a white finish, is this what you are suggesting? This is the *only* way that anything you've asserted is remotely true, which of course, now is a durability discussion of coatings.
See the post above: it plainly shows the images of IP14 and IP15 "whites". The IP15 is much more cream colored, because, just like the blue and black coatings, it has been purposely designed to perform that way in terms of spectrum absorbed/reflected. Apple likely will not release the exact formula ever, but it doesn't require disclosure to know that it's not 100% Titanium Dioxide.
Your car is likely made of a mix of metal, plastics, and bondo....yet, somehow, they manage to produce them in all sorts of colors...not a coincidence or anything functionally different.
Oh yeah, the process of oxidation of pure (e.g. uncoated) Titanium -> Titanium Dioxide, in a regular environment (read: not the 1200 degree autoclaves used to accelerate this process under pressure) is on the scale of *geologic time* to form even a mild white color, that, actually, itself is easily removed by wear, too. So, maybe you are right in about 10 million years, all our iPhones will be pure white by the process you describe (but, unfortunately, they will now also be incredibly brittle, because, well oxidation....)
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