  BL Lacertae has been the target of four observing campaigns by the Whole
Earth Blazar Telescope (WEBT) collaboration. In this paper we present $UBVRI$
light curves obtained by the WEBT from 1994 to 2002, including the last,
extended BL Lac 2001 campaign. A total of about 7500 optical observations
performed by 31 telescopes from Japan to Mexico have been collected, to be
added to the $\sim 15600$ observations of the BL Lac Campaign 2000. All these
data allow one to follow the source optical emission behaviour with
unprecedented detail. The analysis of the colour indices reveals that the flux
variability can be interpreted in terms of two components: longer-term
variations occurring on a few-day time scale appear as mildly-chromatic events,
while a strong bluer-when-brighter chromatism characterizes very fast
(intraday) flares. By decoupling the two components, we quantify the degree of
chromatism inferring that longer-term flux changes imply moving along a $\sim
0.1$ bluer-when-brighter slope in the $B-R$ versus $R$ plane; a steeper slope
of $\sim 0.4$ would distinguish the shorter-term variations. This means that,
when considering the long-term trend, the $B$-band flux level is related to the
$R$-band one according to a power law of index $\sim 1.1$. Doppler factor
variations on a "convex" spectrum could be the mechanism accounting for both
the long-term variations and their slight chromatism.
