2004BaltA..13..253S

We found that color indices of 23 optical afterglows (OAs) of GRBs show negligible variations during the decline of brightness for t-T₀<10 days. The colors concentrate at (B-V)₀=0.45±0.15, (V-R)₀=0.41±0.09, (R-I)₀= 0.46±0.13 and (I-J)₀=0.88±0.15, but with a considerable scatter in (U-B)₀. Large color variations occurred only in the OA of GRB 030329 for t-T₀>20 days and they suggest a rapid spectral evolution of the underlying supernova SN 2003dh. During the first ten days, the colors of the OA of GRB 030329 were consistent with those of the other OAs; they suggest that the synchrotron component was initially dominant in the spectrum. We also argue that either the GRB triggers of the OAs analyzed here are not deep inside the star-forming regions of the host galaxies or the dust is destroyed by the initial energetic flash.