It is invisible in a dark room. As the temperature increases, the blackbody spectrum peak shifts to shorter wavelength infrared light, and our skin and nervous system first detect the radiation emitted from the warming metal. When the metal's temperature approaches 1000 Kelvin, our eyes begin seeing a dim, red glow. The change in color is due to the blackbody spectrum shifting to shorter wavelengths with the increase in temperature. Your eye is crudely measuring the temperature of the star - the above color sequence is one of decreasing temperature.
Stars are, to a good approximation, blackbody radiators.
However, keep in mind that the apparent subtle colors of the planets is not a temperature effect. The visible light from planets is due to reflected sunlight, and any coloration you perceive is due to the absorption of certain wavelengths of light, as discussed above. However, I hope to have provided you with a flavor of what goes on.