Long gamma-ray bursts in the Swift era: the color indices of their optical afterglows

We present an analysis of optical afterglows (OAs) of long GRBs which were mostly discovered by NASA Swift. We used the method of the color indices that uses the commonly available multiband photometry. We show that for most OAs, in the observer frame, these colors, corrected for the reddening inside our Galaxy, display prominent clustering in some color-color diagrams