I finally found the Calvin & Hobbes strip I'd been looking for.
It's in Scientific Progress goes "Boink" (on page 23, for
those Robert Rankin fans among us ...)
Calvin: Dad, how come old photographs are always black and white?
Didn't they have colour film back then?
Dad: Sure they did. In fact those old photographs *are* in
colour. It's just the *world* was black and white then.
Calvin: Really?
Dad: Yep. The world didn't turn color until sometime in the
1930s, and it was pretty grainy color for a while, too.
Calvin: That's really weird.
Dad: Well, truth is stranger than fiction.
Calvin: But then why are old *paintings* in color? If the world was
black and white, wouldn't artists have painted it that way?
Dad: Not necessarily. A lot of great artists were insane.
Calvin: But how could they have painted in color anyway? Wouldn't
their paints have been shades of grey back then?
Dad: Of course, but they turned colors like everything else did
in the '30s.
Calvin: So why didn't old black and white photos turn color too?
Dad: Because they were color pictures of black and white, remember?
