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A color-color diagram (CCD) is a graph plotted on two color indices of an astronomical object, e.g., B-V versus U-B. It is a useful way to characterize stars, clusters and galaxies when the distance, and thus the absolute magnitude, is not well known, since ratios of colors are independent of distance (though still dependent upon any reddening). Color-color diagrams have been created for extra-solar planets to help classify them. Colors of transiting planets can be deduced with sufficient observational data.
Note the abbreviation CCD is more often used for charge-coupled device.