Additive Color Synthesis is the method of creating color by mixing various proportions of two or three distinct stimulus colors of light. These primary colors are commonly red, green, and blue, however they may be any wavelengths to stimulate distinct receptors on the retina of the eye.
The distinguishing features of additive color synthesis are that it deals with the color effects of light rather than with pigments, dyes, or filters, and that the stimuli come from separate monochromatic sources. The most common example of additive color synthesis is the color television screen, (or RGB monitor), which is a mosaic of red, green, and blue phosphor dots; at normal viewing distances the eye does not distinguish the dots, but blends or adds their stimulus effects to obtain a composite color effect.
This is an enlarged example of additive color synthesis from a RGB type source.


The principals of additive color synthesis are as follows (numerals indicate relative proportions).

(a) Equal stimulus proportions of two primary colors create a secondary color

1 Red+1 Blue=Magenta
1 Blue+1 Green=Cyan
1 Green+1 Red=Yellow

(b) Equal stimulus proportions of all three primaries create white:

1 Red + 1 Blue + 1 Green = White

(c) Unequal proportions of two or three primaries create other colors:

2 Red+1 Green=Orange
2 Green+1 Red=Lime
1 Blue+1 Green+4 Red=Brown

All color sensations can be produced this way, including those red-blue mixes (purples and magentas) not found at any wavelength band in the spectrum.
In photography, the principles of additive color synthesis underlie making separation negatives for photomechanical reproduction of color images, and dye transfer and similar printing processes. It was also the principal behind the Autochrome film process and similar screen processes. In the darkroom, additive color printing uses red, green, and blue exposures to obtain prints from color negatives and transparencies. ( 1 ) The Grainmaker filter relies on this principle of additive color printing.

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( 1 ) ICP Encyclopedia of Photography.

( autochrome )( color )( film )( grainmaker specifics )( screens )( subtractive )