It surely seems improbable that most of people in the 1920s were dreaming black and white while today 80% dream in color.

If this study occurred in the US then it isn't so improbable. In the 1920s the primary form of entertainment were black and white movies. This might have had enough influence that many of the people who would have had dreams in color had substantial parts of those dreams in "color" but the only relevant colors were black and white. (This notion is partially inspired by my own dreams- I dream in color, but occasionally cartoon characters show up, and when they do, they look like they would in the cartoon even as they interact with normal people, or something sort of like that. So it isn't implausible to me that something similar could happen with black and white characters.)