Ways to Create Colors in a Photograph There are two ways to create colors in a photograph. One method called additive, starts with three basic colors and adds them together to produce some other color. The second method called subtractive, starts with white light(a mixture of all colors in the spectrum(光谱))and by taking away some or all other colors, leaves the one desired. In the additive method, separate colored lights combine to produce various other colors. The three additive primary colors are green, red and blue (each providing about one-third of the wavelengths in the total spectrum). Mixed in varying proportions, they can produce all colors. Green and red light mix to produce yellow, red and blue light mix to produce magenta (a purplish pink) green and blue mix to produce cyan (a bluish green). When equal parts of all three of these primary-colored beams of light overlap(重叠), the mixture appears white to the eye. In the subtractive process colors are produced when dye(染料) absorbs some wavelengths and so passes on only part of the spectrum. The subtractive primaries are cyan, magenta and yellow these primaries or dyes absorb red, green and blue wavelengths respectively, thus subtracting them from white light. These dye colors are the complementary colors to the three additive primaries of red, green and blue. Properly combined, the subtractive primaries can absorb all colors of light, producing black. But, mixed in varying proportions, they also can produce any color in the spectrum. Whether 3 particular colors is obtained by adding colored lights together or by subtracting some light from the total spectrum, the result looks the same to the eye. The additive process was employed for early color photography. But the subtractive method, while requiring complex chemical techniques, has turned out to be more practical and is the basis of all modern color films. What does the passage mainly discuss?