the difference between the apparent photographic magnitude and the apparent visual magnitude of a star.
the difference between the magnitudes of a star in any two spectral regions.
Origin of color index
First recorded in 1920–25
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for color index
The color-index is thus almost invariably low, the average being about 0.5.A Manual of Clinical Diagnosis|James Campbell Todd