The color gamut determines the range of colors that the monitor displays, and the color gamut coverage rate is an important parameter in the quality evaluation of LCD monitor. As brightness is one of the three attributes of color, it should be considered when the color and color gamut are analyzed in 3D color space. The effect of the brightness of LCD backlight on color gamut coverage and color saturation is analyzed, and the effect of the color saturation on the color gamut coverage is also analyzed. 3D color gamut coverage is obtained by the different formulas in the different color spaces using MATLAB. The effect of the brightness on the color gamut area and chroma is analyzed and compared. The results show that CIEDE2000 formula is most suitable to evaluate the color of monitor among CIEDE2000 formula, CIEL*a*b*formula and CIEL*u*v*formula.