  Context. This observational paper has been prepared in the context of the
large multi-wavelength effort by many observers with the aim of following up
the transient flaring event of V404 Cygni that took place for several weeks in
2015 June. Aims. Our main original aim was to contribute to the study of this
transient source by acquiring broad-band photometric observations during its
most active flaring phases. Nevertheless, after a detailed analysis of the
data, several interesting results were obtained that encouraged a dedicated
publication. Methods. The methodology used was based on broad-band differential
CCD photometry. This outburst of V404 Cygni rendered the source a very bright
target easily within reach of small educational telescopes. Therefore, the 41
cm telescope available at the Astronomical Observatory of the University of
Ja\'en was used in this work. Results. We detected variability at different
time scales, both in amplitude and colour. Individual optical flares appear
every half hour on average during our 3 h long observation, although
large-amplitude (\sim 1 mag) variations are also observed to occur on intervals
as short as 10 minutes. Also, colour variations appear to be highly correlated
in a colour-colour diagram. Another remarkable finding is the detection of time
lag, from about one to a fraction of a minute between light curves in different
filters (VRcIc). Conclusions. The observed behaviour is tentatively interpreted
in an scenario based on the ejection of non-thermal emitting, relativistic
plasmons, with their synchrotron spectra extending up to optical wavelengths.
This would render some of the V404 Cygni flares very similar to those of the
well-know microquasar GRS 1915+105
