Chapter 7 - Additive color mixing

Abstract

Additive color mixing is done in each pixel on the screen of any device; lights of three colors are mixed to generate the perception of many other colors, even if physically there are really only three: the additive primaries red, green, and blue, abbreviated RGB. These primaries can be added to one another and the resulting color can be identified on a computer or on the color triangle, where the fractions of red and green are plotted. The color triangle displays hue and saturation but not brightness, thus all low-brightness colors, such as brown, gray, and dark green, are located in the same positions as their high-brightness counterparts. Given three RGB numbers, you can calculate where the color is on the color triangle, and thus deduce which color it is. First, you calculate the fractions of red and green and place a point at those coordinates on the triangle. Then, you trace a line from white (W) to that point, continuing to the edge of the triangle. Where the line intercepts the edge you read the hue: R, G, B, C, M, Y, or intermediate colors, called bluish-M, reddish-M, reddish-Y or orange (O), greenish-Y or lime green, greenish-C or aqua, or seafoam green, and bluish-C. Finally, you determine the saturation by looking at the distance from W, with the farthest, the middle, and closest to white having high-, medium-, and low-saturation, respectively. Complementary colors are on opposite sides of W on the color triangle. These are RGB and CMY, so that R is complementary to C, G to M, and B to Y. Almost all spectral colors, with the exception of RGB, are outside the color triangle, and they have greater saturation than any color in the triangle. The latter are the colors that can be obtained by RGB mixing. Thus, the area of human color vision is greater than the area of the triangle. M and purple (P) are nonspectral colors; they do not exist in the natural visible light spectrum, thus they are not associated with a single wavelength, but two or more wavelengths. M is only a mixed color, and results from the additive mixing of R+B=M. P is dark M, or low-brightness M.