The color you perceive is a weighted average of the intensities of different wavelengths of light that hit your eye. Ultra-violet and infra-red have a weight of

The color you perceive is a weighted average of the intensities of different wavelengths of light that hit your eye. Ultra-violet and infra-red have a weight of zero (invisible). Most colors not at the edge of visible spectrum have weights that are roughly the same order of magnitude. Hot bodies emit various wavelengths, but the distribution of intensities is roughly a bell curve. If you sum up all these intensities, but weight them by the aforementioned weights in the previous paragraph, you would calculate what color would be perceived when looking at that object. Barely glowing hot objects emit a ton of infra-red (with perceptual weight zero) and a bit in the visual spectrum, on the red end of it (with a non-zero weight). So we perceive a red glow. Extremely ludicrously hot objects emit a ton of ultra-violet, X-ray and gamma rays (all with zero weight for visual perception), but also a bit on the blue end of the visible spectrum (with a non-zero weight for perception). So they look blue. Well, they would have looked blue, if the invisible bombardment of gamma rays did not immediately evaporate you.