  We present simultaneous multi-color optical photometry using ULTRACAM of the
transiting exoplanet KIC 12557548 b (also known as KIC 1255 b). This reveals,
for the first time, the color dependence of the transit depth. Our g and z
transits are similar in shape to the average Kepler short-cadence profile, and
constitute the highest-quality extant coverage of individual transits. Our
Night 1 transit depths are 0.85 +/- 0.04% in z; 1.00 +/- 0.03% in g; and 1.1
+/- 0.3% in u. We employ a residual-permutation method to assess the impact of
correlated noise on the depth difference between the z and g bands and
calculate the significance of the color dependence at 3.2{\sigma}. The Night 1
depths are consistent with dust extinction as observed in the ISM, but require
grain sizes comparable to the largest found in the ISM: 0.25-1{\mu}m. This
provides direct evidence in favor of this object being a disrupting low-mass
rocky planet, feeding a transiting dust cloud. On the remaining four nights of
observations the object was in a rare shallow-transit phase. If the grain size
in the transiting dust cloud changes as the transit depth changes, the
extinction efficiency is expected to change in a wavelength- and
composition-dependent way. Observing a change in the wavelength-dependent
transit depth would offer an unprecedented opportunity to determine the
composition of the disintegrating rocky body KIC 12557548 b. We detected four
out-of-transit u band events consistent with stellar flares.
