Oh, this is really neat, this is the same thing they did in Sh! The Octopus in 1937 to do this transformation scene. In black & white, the color of your light can hide makeup, then all you have to do is flip the color, and the audience just sees the difference in the light levels, but cannot see the color shift. The quick explanation for why this works is the blue makeup absorbs red light, looking very dark when only red light is present, but also looks about the same as relatively fair skin when only blue light is present. Same goes for the colors the other way.
Ahh, I think that they had this in one of the early Jekyll and Hyde adaptations as well. :)
Twilight Zone used this trick too, in The Howling Man, where this guy morphs into the Devil.
And again in Long Live Walter Jameson, where this guy loses his artificial youth
I think the first time it was used was Dr Jekyll and Mr Hyde in 1931.
