It's usually a sign of a flashback, where the colors are artificially shifted to distinguish between "normal time" and "memory." I've heard that people tend to dream in black-and-white, or maybe that's just the way they remember dreams. (Every so often I have some unusually colorful moment in a dream, and it's so surprising that I wake up thinking about it; otherwise it's the same ole.)
But during the early developmental stage of Japanese animation, series were in fact created and painted in grayscale. There are many Astro Boy cels that are in monochrome grayscale, and these are especially desirable.
"Flashback" cels that I have include:
Hikaru's first meeting with Umi and Fuu in the OVA
(Interestingly, the figure is monochrome, but the eyes are the normal ruby-red, making for an especially dramatic effect.)
Syaoran as a child learning martial arts
(This is is a red monochrome effect, but the same idea.)