From what I've noticed over the years, frames that are anodized here in the US do not use any dye when they are trying to match stainless slides. The aluminum is left it's natural color and is then clear anodized. They start off looking a dull light grey. I suspect that the process is subject to discoloration over time, be it ultra violet degradation, internal corrosion or voodoo.
Frames anodized in Europe, such as early Italian Inox Berettas, seem to have a grey or silver dye added, which hides the natural color (or changes) of the aluminum and gives the frame a much nicer appearance.
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