There are two types of color; Additive and Subtractive:
Additive colors are the colors that when combined create white. This type of color is defined by physics: Properties of light, Prisms and refraction, frequency. Examples of Additive color are Pixels on a computer screen, television screens, colored lights in a theater. The primary colors of this type are Red, Green, and Blue.
Subtractive colors are the type when you add them together you lose color value and saturation. Remember finger painting or playing with play-doh? Do you recall how easy it was to create "mud" or a grayish drab color? Mix any few colors together and you will get gray. That is subtractive color. Pigment based colors such as in dye and paint are subtractive colors. The primary colors of this type are Red, Yellow and Blue.
As you can see in the image to the left, the values of the three primary additive colors (RGB) can recreate the process colors (CMY) where they overlap. As part of the physics and properties of light, Blue and Green will recreate Cyan, Blue and Red will naturally create Magenta, and Red and Green will naturally create Yellow. The following example images show how adding colored lights to a single object in a scene can change the shadow colors as well as the highlighted areas on the floor surface.
The 3 examples above show black shadows behind each of the spheres in the scene. Since there is no colored light in that area on the canvas, the absence of light creates a black area. The single light source (red, green or blue) is blocked from casting light onto the surface from the sphere. However, in the next set of examples, you will see the effect of a secondary light source added to the scene. The additive values of each of the colored lights build on each other and reveal a very interesting process.
The first example shows how a green light and a red light affect one another when they have overlapping beams. In the overlapped area of the colored lights, you can see that the light color that has been created is yellow. You can see that in the shadow of the red side of the image, there is a green area behind the sphere. The same is true in the green light's shadow - there is a red area from the red light that is casting into that area of the scene.
The red and blue lights in this example to the left show the same effect. Due to the position of the blue light, there is no overcast of the shadow, so there is no black. There is still a red shadow in the blue light's beam, and there is a blue shadow in the red's beam. Notice the overlapping colors create a magenta color on the platform below the sphere.
In the third example, we see the blue and green lights and the colors their beams cast as both shadowed and lit areas. The blue beam leaves a green shadow, and the green beam leaves a blue shadow. As with the prior examples, the overlapping color creates a new color, in this case it is cyan.
The final example shows all three lights casting colored beams onto the sphere. The three shadows behind the sphere show the three examples of the print colors; Cyan, Magenta and Yellow. The overlap of the three colors create a white light on the platform, just below the sphere. The results are similar to the color example at the top of the page showing how additive light, Red, green, and Blue, can create the CMYK color palette used in the printing process
There are a specific set of subtractive colors classified as "Process Colors" which are used as the 4 main colors in offset printing. The "primary" colors used in printing are Cyan, Magenta, Yellow, and Black; also known as C. M. Y. and K. The "K" is not k for blacK, but rather it is "K" as in "Key plate", or the printing plate in which the other three colors are aligned to.
The different colors are printed as separations (single "plates" of each of the colors) using a halftone pattern in succession starting with yellow, then magenta, then cyan, and then finally black to recreate the full color spectrum seen with the eye. Each color separation is burned (developed) onto a plate at a different angle so that the colors will overlap just slightly to help create the illusion of a full color image. The image below shows a representation of the halftone patterns, at different zoom levels.
If you take a magnifying glass and view a piece of newsprint, you will see that the color images are actually comprised of an array of only 4 differnt colored dots.