What is the temperature of a star?

The Simple Answer :

The color of a star's visible light indicates its surface temperature: a relatively cool star glows red (longer wavelength), and very hot ones glow bluish-white or even blue (shorter wavelengths).

• Blue more than 30,000 °Kelvin
• Blue to blue white 10,000 to 30,000 °Kelvin
• White 7,500 to 10,000 °Kelvin
• Yellowish White 6,000 to 7,500 °Kelvin
• Yellow 5,200 to 6,000 °Kelvin
• Orange 3,700 to 5,200 °Kelvin
• Red 1,000 to 3,700 °Kelvin
• Brown less than 1,000 °Kelvin
  
• Black close to 0 °Kelvin

The Technical Definition

The relationship is λmax*T=2.898*10^-3m°K

Where λmax is the wavelength at which the star emits the maximum amount.

So, for example for the Sun λmax = 5.1 nm

Therefore T=(2.898*10^-3m°K)/(5.1*10^-9m)

T=568,000 °C

So as an object increases in temperature, its colour will slowly change, starting at red and progressing into blue and beyond.