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About Color and Color Spaces

A color in Quartz is represented by a set of values. The values are meaningless without a color space that dictates how to interpret color information. For example, the values in Table 4-1 all represent the color blue at full intensity. But without knowing the color space or the allowable range of values for each color space, you have no way of knowing which color each set of values represents.

If you provide the wrong color space, you can get quite dramatic differences, as shown in Figure 4-1. Although the green color is interpreted the same in BGR and RGB color spaces, the red and blue values are flipped.

Color spaces can have different numbers of components. Three of the color spaces in the table have three components, while the CMYK color space has four. Value ranges are relative to that color space. For most color spaces, color values in Quartz range from 0.0 to 1.0, with 1.0 meaning full intensity. For example, the color blue at full intensity, specified in the RGB color space in Quartz, has the values (0, 0, 1.0). In Quartz, color also has an alpha value that specifies the transparency of a color. The color values in Table 4-1 don’t show an alpha value; for more details, see “The Alpha Value.”

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