  We have observed several relatively deep transits of the white dwarf WD
1145+017 with the GTC over the wavelength range 480 nm to 920 nm. The
observations covered approximately one hour on 2016 Jan 18 and two hours on
2016 Jan 20. There was variable extinction of the white dwarf during much of
that time, but punctuated by four sharp transits with depths ranging from 25%
to 40%. The spectrum was dispersed with a grism, and the flux data were
ultimately summed into four bands centered at 0.53, 0.62, 0.71, and 0.84
$\mu$m. After careful normalization, we find that the flux light curves in all
four bands are consistent with being the same, including through the deepest
dips. We use these results to compute \AA ngstr\"om exponents, $\alpha$, for
the particles responsible for the extinction and find |$\langle \alpha \rangle
$| $\lesssim 0.06$, assuming that the extinction is relatively optically thin.
We use the complex indices of refraction for common minerals to set constraints
on the median sizes of possible dust grains, and find that for most common
minerals, particle sizes $\lesssim 0.5 \, \mu$m can be excluded.
