BW filmmakers were, of course, well aware that they were filming in BW and would select colors for sets and costumes that would look good in BW. For example, chocolate milk was used for blood. You wouldn't want a colorizer to determine the actual colors, but the intended colors!

Even today, directors rarely seem to want to film in actual color. They'll tint everything sepia, or that hideous blue-orange scheme that is so popular these days.

http://priceonomics.com/why-every-movie-looks-sort-of-orange...