A visual guideline for applying makeup during the early days of film.
Orthochromatic film stock was the standard during the birth of film, mostly due to price. However, it is insensitive to red and yellow light on the spectrum. The process to correct this insensitivity to red and yellow makes it oversensitive to blue and violet.
Because of this, actors had to be made up with highly exaggerated and contradictory colors in order to look “natural” on film.