  We monitored BL Lacertae simultaneously in the optical B, V, R and I bands
for 13 nights during the period 2012-2016. The variations were well correlated
in all bands and the source showed significant intraday variability (IDV). We
also studied its optical flux and colour behaviour, and searched for inter-band
time lags. A strong bluer-when-brighter chromatism was found on the intra-night
time-scale. The spectral changes are not sensitive to the host galaxy
contribution. Cross-correlation analysis revealed possible time delay of about
10 min between variations in the V and R bands. We interpreted the observed
flares in terms of the model consisting of individual synchrotron pulses.
