Colors and Polarity

Here's a puzzle. If we look at colors, we see, as expected, that black and white are more common than the primaries, and the primaries are more common than any other color terms. But something unexpected has happened roughly since the early sixties: black and white together exhibited a rise in prevalence that is independent of the color terms. Has our thinking become more polarized?

(Note: there is an unfortunate Stroop effect here. The curves for black and white are not in black and white. Red, blue, green and yellow are drawn in their own colors. Brown is a light green. Sorry).