I think there are two problems with the physics book question...
(1) It says "pure" green and "pure" blue pigments are mixed.
I don't know exactly what that means. Presumably it means
the reflected light (the non-absorbed light) is not very
broadly distributed.
If we assume the green pigment's reflected spectral distribution
is centered on 520 nm, we have no idea whether this pigment
reflects light from 510 to 530, or from 450 to 590, or what.
If, for example, the green had considerable reflection down
to about 470, and the blue had considerable reflection up to
about 500, then the overlap region would indeed appear cyan
to the average person.
However, a blue pigment that extends to 500 is not something I
would call "pure blue," and a green pigment that extends to 470
is not something I would call "pure green."
I would indeed answer the question as "black" and I assume
(I hope) the answer key is a misprint.
(2) I wish we could refrain from using red, blue, green in
reference to pigments when we are discussing or trying to teach
color mixing.
Although I agree there are pigments that are red (and blue, and
green) we don't use those pigments for mixing. For subtractive
color mixing we use cyan, magenta, yellow, and black. It just
clouds the issue to talk about blue, red, green pigments if we are
also discussing mixing.
I had hopes that as more people got experience with color
inkjet printers, and especially now that digital cameras are
becoming popular, people would learn that CMYK (cyan,
magenta, yellow, black) are the ink cartridge pigments they need
to print color photographs. Good color photograph printers
now use 6 pigments... cyan, light cyan, magenta, light magenta,
yellow, and black. The addition of the light cyan and light magenta
allows them to do pale-colored regions without relying on
dithering as much.
I thought people would eventually get used to cyan, magenta, and
yellow as the "primary pigments" (primary subtractive colors)
and use red, green, blue as the primary additive colors.
Unfortunately, many people look at the cyan ink and call it blue,
and they look at the magent ink and call it red (or perhaps pink).
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu