When an Astronomer Measures a Color Index for a Star

When an astronomer measures a color index for a star, what is she measuring?

A) what color the human eye sees when people look at that star
B) the difference between how bright a star looks at two different wavelength regions
C) the total luminosity of the star in all parts of the electromagnetic spectrum
D) the amount of hydrogen in the atmosphere of the star
E) how the color of the star is changed when its light passes through the Earth's atmosphere
Section 17.3: The Spectra of Stars (and Brown Dwarfs)