Strictly speaking, the materials used to make lens coatings have no colors of
their own -- they are clear and colorless. In this literal sense, the pompous
salesman is correct.
However -- the coatings are applied in layers thinner than a wavelength of light
in order to create an "impedance match" between air and glass, so that less
light is reflected.
The degree of matching varies with wavelength, it being "perfect" (that is,
preventing all reflection) only at the wavelength where the coating is 1/4-wave
thick. It is less-effective at other wavelengths, which is why the reflection
seems to have a color. Single-coated lenses look purplish, because more light is
reflected at red and blue wavelengths than at green.
Multicoating uses several layers to get better matching over a wider range of
wavelengths. These apparently provide more correction at the longest and
shortest wavelengths, leaving a net greenish reflection. Note, however, that MC
lenses show less total reflection than SC lenses.
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