Alex, I think you've just got proof to your theory. I've just done a little bit of research on anodizing aluminium. Having taken the watch apart to clean I think the inner rotating bezel is constructed from aluminium and anodized to get the colors. After that the numbers are transfer inked from a rubber pad. It is possible that anodization on aluminium could discolor over time after the effects of inadequate sealing of the coating and or prolonged exposure to UV light. There are some cases even where black anodizing turned pink! In a way I'm actually glad that the bezels on my GMTs turned tan than pink!