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In the early days of movies the monochrome film they used was notoriously sensitive to the blue end of the spectrum, including ultraviolet, to such an extent that clouds in the sky didn't show on the film at all. As skin tones are at the red end of the spectrum humans appeared darker than was natural, and uneven skin tones looked dirty. To overcome this actors wore heavy white (or orange if they wanted to appear dark-skinned) greasepaint-and-powder make-up, painted on eyebrows, and eye make up for definition, and hence appeared 'ugly' to the human eye but looked good to the camera.

Find some background here and here.