  We present UBVRI light curves of BL Lacertae from May 2000 to January 2001,
obtained by 24 telescopes in 11 countries. More than 15000 observations were
performed in that period, which was the extension of the Whole Earth Blazar
Telescope (WEBT) campaign originally planned for July-August 2000. Rapid flux
oscillations are present all the time, involving variations up to a few tenths
of mag on hour time scales, and witnessing an intense intraday activity of this
source. Colour indexes have been derived by coupling the highest precision B
and R data taken by the same instrument within 20 min and after subtracting the
host galaxy contribution from the fluxes. The 620 indexes obtained show that
the optical spectrum is weakly sensitive to the long-term trend, while it
strictly follows the short-term flux behaviour, becoming bluer when the
brightness increases. Thus, spectral changes are not related to the host galaxy
contribution, but they are an intrinsic feature of fast flares. We suggest that
the achromatic mechanism causing the long-term flux base-level modulation can
be envisaged in a variation of the relativistic Doppler beaming factor, and
that this variation is likely due to a change of the viewing angle. Discrete
correlation function (DCF) analysis reveals the existence of a characteristic
time scale of variability of about 7 h in the light curve of the core WEBT
campaign, while no measurable time delay between variations in the B and R
bands is found.
