Richard
The printer would need to know the correct colors. For example, in the Fool the sky is yellow, and the sun is white, because of the color symbolism of what the sky and the sun represent. (Compare this with the colors in the Sun card.) I don't think the printer would have guessed this without directions, and to avoid ambiguity, it seems likely that they had colored drawings to work from rather than written instructions. It may have been otherwise, but this seems plausible to me.