Color management systems are necessary because the way we usually define colors is ambiguous. Most applications represent color using either the RGB (red, green, blue) or CMYK (cyan, magenta, yellow, black) model. Both RGB and CMYK are referred to as device-specific or device-dependent color models, because the actual color produced depends on the behavior of the device producing it. Device-specific color spaces represent color using numerical values that indicate the amount of each colorant a device uses to produce color. RGB colors may indicate the signal strength generated by a scanner's sensor when it records light through the red, green, and blue filters, or they may represent the voltage sent to the guns that excite the monitor's red, green, and blue phosphors. CMYK colors specify the amount of each ink laid down to produce a particular color. The problem is that the same RGB or CMYK values will produce a different color on different devices. To keep the color consistent across different devices that use different color spaces, we need a way of transforming the device-specific RGB or CMYK values so that they produce the correct color on each device. This is what a color management system does.