And, yes, this took a long time.
Armin Vit in Dark and Fleshy: The Color of Top Grossing Movies, on the design site Under Consideration, commenting on the exercise in which he maps out the dominant color palettes in historically high-grossing films.
His conclusion: The movie posters for the highest-grossing NC-17, R, PG-13 and PG movies (with one exception -- Shrek) are all dominated by the color black, a color that is generally "feared" by design clients as being too dark and gloomy.
Flesh tones came in second. (Flesh is not so feared because baby, sex sells.)
What's missing from this analysis, of course, is the distribution of black across all the other movie posters. Without knowing that, it's dicey to conclude that correlation implies causation.
But it's kinda fun to speculate »