  We monitored BL Lacertae in the B, V, R and I bands for 14 nights during the
period of 2016-2018. The source showed significant intraday variability on 12
nights. We performed colour-magnitude analysis and found that the source
exhibited bluer-when-brighter chromatism. This bluer-when-brighter behavior is
at least partly caused by the larger variation amplitude at shorter wavelength.
The variations at different wavelengths are well correlated and show no
inter-band time lag.
