Published online by Cambridge University Press: 22 July 2013
We show the power of the method of the color indices that uses the commonly availablemultiband photometry of optical afterglows (OAs) of long GRBs detected bySwift. Our study shows that for most OAs, in the observer frame, thesecolors, corrected for the Galactic reddening, display prominent clustering in thecolor-color diagrams. The color indices enable us to distinguish OAs from other types ofobjects even without available gamma-ray emission. The color-color diagrams can be used toassess the extinction inside the host galaxy. Our approach is also generally important forinvestigating the numerous faint OAs for which obtaining spectra with good S/N ratio isoften impossible.