Until the 1990s, the concept of color mixing of light (as opposed to pigment) as we know it today was usually relegated to borderlights and cyc lighting. Gel filters produced the three primary colors of light: RED, BLU, and GRN, which were placed in alternating lamps or cells. Often AMB was added as a fourth color to expand the range, or for a daytime cyc scene, DAYLT BLU. Today we refer to mixing a color using the primarly colors of light as "additive color mixing."
In kindergarten we all learned when mixing finger paints: blue and yellow make green, yellow and red make orange, blue and red make purple. Things would be so much simpler if only we had never been taught that abomination! In actuality, the three primary colors of pigment are CYAN, MAGENTA, and YELLOW, and, with BLACK added: CMYK. Anyone who has ever changed an inkjet printer's cartridge knows this. Workers at printing plants knew this, but apparently the word never got to the kindergarten teachers. Creating a color by mixing pigment is known as "subtractive color mixing." Modern moving lights use dichroic filters (which reflect rather than absorb unwanted color) instead of pigment, but the principle is the same. It's interesting to note that even in the old days, when we put a red filter into a light, we were "subtractive mixing," in that we were subtracting all colors except red from the previously white light.
Today, we often still have the RED, BLU, and GRN cyclights, but the preponderance of LED fixtures have changed the order of the colors, and greatly expanded the possibilities. The simplest LED fixture uses three colors: RGB. All have difficulty mixing a white and light pastels, so often either a WHT or AMB circuit is added. One manufacturer, Selador, goes so far as to use SEVEN colors (Red, Red-Orange, Amber, Green, Cyan, Blue, Indigo) to compensate for those lacking in an RGB system. In a three-color system with each color having the range of 0 (no saturation) to 255 (maximum saturation), a theoretical maximum exists of 256^3 (~16 million). A seven color system produces 256^7 (~72 quadrillion). A lighting designer is unlikely to use more than twenty discrete colors in any particular production, but more colors equal a smoother crossfade from one color to another, something that all scrollers cannot do (with the exception of the Morpheus ColorFader™).
In summary, RGB is additive mixing, CMY is subtractive. Some consoles allow the user to think of the colors in either manner, simply by inverting the values for one system or other.
Draw a color wheel putting the three primaries of either system at 120° (Noon, 4 o'clock, and 8 o'clock). Mix any two adjacent colors, resulting in the three secondary colors. For any given color, note that the primary of the opposite system is directly across. These two would be "complimentary" colors and will also, theoretically at least, mix to white. This brings us back to the 1932 Stanley McCandless System, where Mac taught us that blue from one side and amber (yellow) from the other would provide maximum plastisticy, while still mixing to white in the center. What values would one use in each system (additive or subtractive) to achieve Roscolux#57 Lavender? R23 Orange? Apollo#7050?
Any discussion of color mixing always includes the disclaimer that results may vary as colors and sources used on the stage are not "pure" enough to produce therotical scientific results. This was true of plastic and glass color media, but dichroics and LEDs are sometimes "too pure" to mix desired colors. In particular, LEDs have a "spike" in their SPD (Spectral Power Distribution) curves at 700 (red), 530 (green), and 470 (blue) nanometers, respectively.
For more, see this thread: http://www.controlbooth.com/forums/lighting/9159-cmy-rgb-values.html. See also this site, or http://www.rgbworld.com/color.html. This interactive site may not work with some browsers.
See also this thread: http://www.controlbooth.com/forums/lighting/9197-amber-vs-yellow.html.
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