The area on a color screen (CRT) in between the phosphor dots, the matrix, does not illuminate. Its color determines what the screen looks like when the power is off--pale grey, dark green-grey, or nearly black. Darker and black matrices gives an impression of higher contrast to the displayed images. Lighter grey matrices make for purer white. The distinctions are subtle, however, and unless you put two tubes side-by-side, you're unlikely to be able to judge the difference. The effect is more pronounced on older screens.