  We present the results of extensive multi-band intra-night optical monitoring
of BL Lacertae during 2010--2012. BL Lacertae was very active in this period
and showed intense variability in almost all wavelengths. We extensively
observed it for a total for 38 nights; on 26 of them observations were done
quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with
an average sampling interval of around 8 minutes. BL Lacertae showed
significant variations on hour-like timescales in a total of 19 nights in
different optical bands. We did not find any evidence for periodicities or
characteristic variability time-scales in the light curves.
  The intranight variability amplitude is generally greater at higher
frequencies and decreases as the source flux increases.
  We found spectral variations in BL Lacertae in the sense that the optical
spectrum becomes flatter as the flux increases but in several flaring states
deviates from the linear trend suggesting different jet components contributing
to the emission at different times.
