  We present the anisotropic optical conductivity of MgB$_{2}$ between 0.1 and
3.7 eV at room temperature obtained on single crystals of different purity by
the spectroscopic ellipsometry and reflectance measurements. The bare
(unscreened) plasma frequency $\omega_{p}$ is almost isotropic and equal to 6.3
eV, which contrasts some earlier reports of a very small value of $\omega_{p}$.
The data suggests that the $\sigma$-bands are characterized by a stronger
electron-phonon coupling $\lambda_ {tr}$ but smaller impurity scattering
$\gamma_{imp}$, compared to the $\pi$-bands. The optical response along the
boron planes is marked by an intense interband transition at 2.6 eV, due to
which the reflectivity plasma edges along the a- and c-axes are shifted with
respect to each other. As a result, the sample spectacularly changes color from
a blueish-silver to the yellow as the polarization is rotated from the in-plane
direction towards the c-axis. The optical spectra are in good agreement with
the published {\it ab initio} calculations. The remaining discrepancies can be
explained by the relative shift of $\sigma$-bands and $\pi$-bands by about 0.2
eV compared to the theoretical band structure, in agreement with the de
Haas-van Alphen experiments. The widths of the Drude and the interband peaks
are both very sensitive to the sample purity.
