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The color of a star is a critical feature to reflect its physical propertysuch as the temperature. The color index is usually obtained via absolutephotometry, which is demanding for weather conditions and instruments. In thiswork, we present an astrometric method to measure the catalog-matched colorindex of an object based on the effect of differential color refraction (DCR).Specifically, we can observe an object using only one filter or alternatelyusing two different filters. Through the difference of the DCR effect comparedwith reference stars, the catalog-matched color index of an object can beconveniently derived. Hence, we can perform DCR calibration and obtain itsaccurate and precise positions even if observed with Null filter during a largerange of zenith distances, by which the limiting magnitude and observationalefficiency of the telescope can be significantly improved. This method takesadvantage of the DCR effect and builds a link between astrometry andphotometry. In practice, we measure the color indices and positions of Himalia(the sixth satellite of Jupiter) using 857 CCD frames over 8 nights by twotelescopes. Totally, the mean color index BP-RP (Gaia photometric system) ofHimalia is 0.750 \pm 0.004 magnitude. Through the rotational phased color indexanalysis, we find two places with their color indices exceeding the mean \pm 3\sigma.

An Astrometric Approach to Measuring the Color of an Object