About Colour/Three Primary Colour Theory page 12 of 15

Not one of the "Secondary" colors of Fig 3 is a pure color. To be a primary color, a color must be a pure color. Today, however, political correctness is more important than technical accuracy. Thus, the "Secondary" colors are now referred to, as Subtractive Primary Colors and the colors of the three spotlights are no longer the "Primary Colors" but now are the Additive Primary Colors.

A set of three Subtractive Primary Colored Filters is shown in Fig 3 to the right. These are partially overlapped on a light table shining a single source of White light through them. Where all three filters overlap, all the colors that make up White light (Red, Green and Blue) are cut off, leaving only Black, - which is the absence of light.