| Abstract : | Optical flux and colour variability have long been a subject of blazar research as they afford a unique perspective on accretion disc and jet physics and might constrain the origin of corresponding brightness variations, even when restricted to the optical band. However, despite several efforts, a clear picture has not been established yet. In order to address any universality in the colour behaviour of the blazars, for the first time, we investigate the largest homogeneous sample taken to date using the multiband light curves from the Zwicky Transient Facility (ZTF DR6) survey. Assessing the colour variability characteristics over ∼2 yr time-scales, we found that 18.5 per cent (84 out of 455) BL Lacs showed a stronger bluer-when-brighter (BWB) trend, whereas 9.0 per cent (41 out of 455) showed a redder-when-brighter (RWB) trend. The majority (70 per cent) of the BL Lacs showing RWB are host galaxy dominated. For the FSRQ subclass, 10.2 per cent (45 out of 442) objects showed a strong BWB trend and 17.6 per cent (78 out of 442) showed a strong RWB trend. Hence, we find that BL Lacs more commonly follow a BWB trend than do FSRQs. We also found many blazars to evince shorter partial trends of BWB or RWB nature (or occasionally both). I will discuss the possible mechanisms contributing to the observed BWB and RWB trends or the complex behaviours seen in the colour behaviours of the blazar population.
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