Intra-day variability of BL Lacertae from 2016 to 2018

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 intra-day variability on 12 nights. We performed color-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 wavelengths. The variations at different wavelengths are correlated well and show no inter-band time lag.