The red line on this plot shows the amount of light emitted per area vs. wavelength, for an object that is 3000 K (the surface temperature of a cool star). Although it emits mostly infrared light, it still emits some visible light. It emits more red light than blue, so it appears red to our eyes.
This is incidentally why old-fashioned incandescent light bulbs are so inefficient: the majority of the light they produce is invisible (to our eyes) infrared light.
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