color tracking 
Each pixel on a CRT monitor's screen consists of red, green, and blue spots of phosphor, each lit up by a separate electron gun. Color tracking refers to a monitor's ability to keep all three electron guns operating at equal strength when displaying different brightness levels. In practice, one gun tends to overpower the other two, resulting in slight reddish, greenish, or bluish hues that are visible in dark-gray areas of an image.