ITU-R BT.1361 defined the range of input signal for gamma correction was [-0.25, 1.33]. This paper calculated the widest color gamut and the corresponding three primary colors under this range for video system based on trichromatic principle using colorbar signal widely used in TV system, developed a kind of nonlinear quantization method to convey this gamut compatibly in current conventional color gamut video system with primary colors defined in ITU-R BT.709, and derivated a new set of encoding scheme with the new primary colors calculated in this paper. The results show that the coverage ratio of widest color gamut in ITU-R BT.1361 for video system based on trichromatic principle is 60.65%, and both of the two schemes can compress this color gamut so that it can be conveyed compatibly in current conventional color gamut video system.