Optical Variability and Colour Behaviour of 3C 345
  The colour behaviour of blazars is a subject of much debate. One argument is
that the BL Lac objects show bluer-when-brighter chromatism while the
flat-spectrum radio quasars (FSRQs) display redder-when-brighter trend. Base on
a 3.5-year three-colour monitoring programme, we studied the optical
variability and colour behaviour of one FSRQ, 3C 345. There is at least one
outburst in this period. The overall variation amplitude is 2.640 mags in the
$i$ band. Intra-night variability was observed on two nights. The
bluer-when-brighter and redder-when-brighter chromatisms were simultaneously
observed in this object when using different pairs of passbands to compute the
colours. The bluer-when-brighter chromatism is a shared property with the BL
Lacs, while the redder-when-brighter trend is likely due to two less variable
emission features, the Mg\,{\sc ii} line and the blue bump, at short
wavelengths. With numerical simulations, we show that some other strong but
less variable emission lines in the spectrum of FSRQs may also significantly
alter their colour behaviour. Then the colour behaviour of an FSRQ is linked
not only to the emission process in the relativistic jet, but also to the
redshift, the passbands used for computing the colour and the strengths of the
less variable emission features relative to the strength of the non-thermal
continuum.
