Black Body ( the spectrum )

As the temperature of a blackbody increases, the wavelength shifts from red ( long wavelength) to blue (shorter wavelength) . Extremely hot blackbody (example suns with 5800 Kelvin ) emits  most of their energy in the ultraviolet range, while cool blackbody emits  primarily in the infrared.The color of an object depends upon its temperature as predicted by Planck's formula.See figure below :

Every object emits thermal radiation but our eyes do not catch it as they only sensitive to a very small portion of the electromagnetic spectrum. See figure below :  

For us to see the radiation,an object must be quite hot for it to emits a significant amount of visible light. The Stefan-Boltzmann allows us to determine the temperature by measuring the color spectrum given off by the heating element. In fact, measuring color is the primary means astronomers have of determining the temperatures of distant stars.