When a metal heats up why is the color of light emitted first red/yellow and not any other colour?
The human eye can see electromagnetic radiation within a narrow range of wavelengths. This is approximately between 400 nm and 700 nm. Light with a wavelength of 700 nm appears red in color and that with a wavelength of 400 nm appears violet. All the other colors have a wavelength between these. Light with a wavelength above 700 nm is referred to as infra-red and that with a wavelength less than 400 nm is called ultra-violet.
Any object at a temperature T above absolute zero emits electromagnetic radiation. The wavelength at which it gives off the maximum electromagnetic radiation is given by Wien's Displacement Law as L = b/T, where b is a constant and T is the temperature of the body.
As the temperature of the body and the wavelength at which it emits the maximum electromagnetic radiation is inversely proportional, an increase in temperature leads to a decrease in the wavelength. This makes the color of light given off first as red and it gradually moves towards violet as the temperature is increased.