Pre-scientific authors tended to assume they dreamed in color, while studies in the first half of the 20th century found very few people who reported dreaming in color. In the 1960s, this consensus was overturned, and recent studies show that today, more than 80% of people report that they dream in color.

I think I once heard someone hypothesize that people in the early/mid-20th century tended to dream in black and white as a result of watching lots of TV/films in black and white.