Basic colour darkroom printing theory

 Color film instead of having one layer of emulsion has three, all of which are basically black and white film emulsion with one key difference. Each of these layers are made sensitive to different wavelengths of light by the introduction of carbo-cyanine dyes in the formulation of the emulsion; different dyes deliver different spectral absorption characterisitcs. The first layer is the blue sensitive layer. This layer is first because all film is inherently sensitive to blue light, and the blue emulsion requires the least amount of filtration. On top of the blue layer is an Ultra-violet filter. Next we find the green sensitive layer, this layer has a yellow filter layer above it and lastly we have the red sensitive layer under a red filter. Unlike black and white film, which has the silver in the emulsion reduced to metallic silver in the development process, color film actually has the silver halides that were exposed to light replaced with color dyes that correspond to the color layer. For example, when the film is processed the red layer has all of it's silver halides that were exposed to red light replaced with red dyes. The intensity of the dyes introduced are directly propotional to the amount of exposure the layer received. Once the film is processed the varying intensities in the three layers sandwiched on top of one another give all the different hues present in the original scene. Once developed the film is then placed in a bleaching bath which removes ALL of the silver in the film. leaving only the dyes.
There are two basic color systems in use today, these are the additive and the subtractive color systems. In the additive color system we work with Red Green and Blue. In the subtractive system we work with the colors Yellow Magenta and Cyan. the two systems derive their names by the manner in which they combine the colors. The additive system can be described as the combination of the three basic colors to achieve any color required. If for instance we add Red and Blue together in equal amounts we will get Magenta a sort of purple color. By varying the amounts of red and blue we get all the colors that reside in the spectrum between red and blue. The same is true with regards to the combination of blue and green and green and red. The interesting thing about the additive system is that if we combine all three colors we get white. This is because white is the presence of all colors.

The subtractive system on the other hand works by the act of cancellation of different light (or subtraction if you Will). If we place three lights, each with a different filter in front of them, one with a magenta filter one with a cyan filter and one with a yellow filter, so they can shine on a wall in a manner in which all three lights will intersect with each other we will see the following. In the area where yellow and magenta combine we will see red at the intersection, likewise we will see green at the point where cyan and yellow combine and finally we will see blue at the point where cyan and magenta combine. Just as there was a unique quality in the additive process there is an even more bizarre quality in the subtractive. If we combine all of the colors of the subtractive process together we will achieve black at the point where all the colors intersect. This is due to the fact that the three colors cancel each other out, as it were, and yied the absence of all color, or black.

This brings up the point of complimentary colors. Complimentary colors refers to the manner in which the two systems interact with one another. For example if I were to print a negative and I found that the whites in the print are too red I can "cancel out" this effect by adding cyan. If you look at the above diagram you will see that Cyan is directly opposite of red and that the two colors point of commonality is black. This is how the cyan is able to cancel out the red. Consequently Yellow and blue are complimentary as are magenta and green. 

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