  We present multi-color optical observations of long-duration gamma-ray bursts
(GRBs) made over a three year period with the robotic Palomar 60 inch telescope
(P60). Our sample consists of all 29 events discovered by Swift for which P60
began observations less than one hour after the burst trigger. We were able to
recover 80% of the optical afterglows from this prompt sample, and we attribute
this high efficiency to our red coverage. Like Melandri et al. (2008), we find
that a significant fraction (~ 50%) of Swift events show a suppression of the
optical flux with regards to the X-ray emission (so-called "dark" bursts). Our
multi-color photometry demonstrates this is likely due in large part to
extinction in the host galaxy. We argue that previous studies, by selecting
only the brightest and best-sampled optical afterglows, have significantly
underestimated the amount of dust present in typical GRB environments.
