We will make use of the blackbody applet below to show why and how color index maps directly into surface temperature.

Wavelength in Angstroms
I N T E N S I T Y

Procedure:

1. Set the temperature to 6000 degrees.
2. Click on the box labeled "Draw Limits of Integration" this will bring up a series of white dotted lines. Notice that one is superposed over the "blue" part of the spectrum and one is superposed over the "green" part of the spectrum. This white lines represent filters that you put in front of a digital camera. The ratio of light received between one filter and another gives us the "color" of the star, which in turn is a direct measure of its surface temperature.

3. We use differences between the Blue (B) and Green (V) filters to measure temperature. Note that for 6000 degrees, the B-V index is 0.58.
4. Change the tempearture to 8000 degrees you will see that B-V is now 0.34
5. Go ahead and change temperatures again. You will see that over the temperature range of 3000 - 20000 degrees, there is a unique value of B-V for each temperature. This is basically how astronomers can determine stellar temperature. When we make our HR diagrams we will use B-V as the X-axis and this simulation tells you approximately what temperature a given B-V index corresponds to.