  We present narrowband images of the gravitational lens system Q~2237+0305
made with the Nordic Optical Telescope in eight different filters covering the
wavelength interval 3510-8130 \AA. Using point-spread function photometry
fitting we have derived the difference in magnitude versus wavelength between
the four images of Q~2237+0305. At $\lambda=4110$ \AA, the wavelength range
covered by the Str\"omgren-v filter coincides with the position and width of
the CIV emission line. This allows us to determine the existence of
microlensing in the continuum and not in the emission lines for two images of
the quasar. Moreover, the brightness of image A shows a significant variation
with wavelength which can only be explained as consequence of chromatic
microlensing. To perform a complete analysis of this chromatic event our
observations were used together with Optical Gravitational Lensing Experiment
light curves. Both data sets cannot be reproduced by the simple phenomenology
described under the caustic crossing approximation; using more realistic
representations of microlensing at high optical depth, we found solutions
consistent with simple thin disk models ($r_{s}\varpropto \lambda^{4/3}$);
however, other accretion disk size-wavelength relationships also lead to good
solutions. New chromatic events from the ongoing narrow band photometric
monitoring of Q~2237+0305 are needed to accurately constrain the physical
properties of the accretion disk for this system.
