The diffraction transmission characteristics of submicrometer gratings (SMGs) designed for coupling tricolor light out of a light guide plate (LGP) are discussed. Three discrete SMGs are designed for three special wavelengths: red \left(700\text{\hspace{0.17em} nm}\right) , green \left(546.1\text{\hspace{0.17em} nm}\right) , and blue \left(435.8\text{\hspace{0.17em} nm}\right) . The propagation direction of the output tricolor light is perpendicular to the surface of the LGP and can pass through the corresponding pixels of liquid crystal. Calculated by the rigorous coupled-wave theory, the first-order transmission efficiency as a function of grating depth is found to be approximate to a sinusoidlike curve and can be utilized to obtain uniform illumination. The theoretical maximum transmission is as much as 44%. The performance of the LGP composed of SMGs is also demonstrated by our experiments. Compared with other types of LGP, the present device has the advantage of flexible control of illumination angle and wavelengths. Additionally, lossy and costly color filters are unnecessary in the proposed configuration.