About coating colors:
A lens coating cannot cover the entire visible spectrum; only part of
it. That's the whole purpose of multi-coating. A single coating is
typically centered within the visible spectrum transmitting the middle of
it better than the far ends. A multi-coating has successive coatings to
improve light transmission more evenly across the entire visible spectrum.
A single coating centered in the visible spectrum will reflect more red and
blue than any other portion of the spectrum because it isn't transmitted
through the lens as well as the colors near the middle. This is why a
prominent purplish reflection should be expected from a single coated
lens. It's also why a multi-coated lens should reflect several colors and
a prominent green should normally be among them.
-- John
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