Two layers of phosphor red and green are coated inside of CRT screen and color depends on how far the beam pentrates into the phosphor layer. A beam having very fast electron penetrates through the red layer and excites inner green layer. A beam of slow electrons excites only outer layer of red phosphor. An intermediate speed beam produce combination of two color: orange and yellow. Hence, color is controlled by beam acceleration voltage. It is inexpensive method but can only produce four color.