The principle of primary-color method for temperature measurement is firstly introduced in this article. Based on the lineal model of spectral emissivity in the range of visible spectrum, the formulation of radiative chrominance of real bodies is put forward. Afterwards, it's made clear that two temperature solutions can be derived from one specific chrominance at most. The radiative color gamut is determined and divided into two components, the single-solution gamut and the dual-solution gamut.