  The spectrum of gamma-ray burst (GRB) afterglows can be studied with color
indices. Here we present a large comprehensive catalogue of 70 GRBs with
multi-wavelength optical transient data on which we perform a systematic study
to find the temporal evolution of color indices. We categorize them into two
samples based on how well the color indices are evaluated. The Golden sample
includes 25 bursts mostly observed by GROND, and the Silver sample includes 45
bursts observed by other telescopes. For the Golden sample, we find that 95\%
of the color indices do not vary over time. However, the color indices do vary
during short periods in most bursts. The observed variations are consistent
with effects of (i) the cooling frequency crossing the studied energy bands in
a wind medium (43\%) and in a constant density medium (30\%), (ii) early dust
extinction (12\%), (iii) transition from reverse shock to forward shock
emission (5\%), or (iv) an emergent supernova emission (10\%). We also study
the evolutionary properties of the mean color indices for different emission
episodes. We find that 86\% of the color indices in the 70 bursts show
constancy between consecutive ones. The color index variations occur mainly
during the late GRB-SN bump, the flare and early reversed-shock emission
components. We further perform a statistical analysis of various observational
properties and model parameters (spectral index $\beta_{o}^{\rm CI}$, electron
spectral indices $p^{\rm CI}$, etc.) using color indices. Overall, we conclude
that $\sim$ 90\% of colors are constant in time and can be accounted for by the
simplest external forward shock model, while the varying color indices call for
more detailed modeling.
