The temperature was almost certainly determined by fitting the spectral data to the black body radiator equation, the reason that red-hot metal glows red. Basic

The temperature was almost certainly determined by fitting the spectral data to the black body radiator equation, the reason that red-hot metal glows red. Basically, objects emit light when they are warm, and the hotter they are, the higher the energy of their emissions (more blue). At normal Earth temperatures, these emissions will be in the infrared, not visible. Remember, the Landsat data includes not just visible data, but also registered images at a bunch of other wavelengths (near and far IR, at least). The actual color of the object is not really relevant.