Actually, this kind of encoding is quite common. Analog television, digital video and JPEG images are all encoded using brightness, red-green and blue-yellow channels. The details vary (eg. between YUV, YCbCr and YPbPr) but they all take advantage of the fact that our eyes are more sensitive to variations in brightness than to variations in colour. As a result, we can subject the colour data to higher levels of compression without noticing any visual degradation of the image.