  We report the flux and spectral variability of PG 1553+113 on intra-night
(IDV) to short-term timescales using BVRI data collected over 91 nights from 28
February to 8 November 2019 employing ten optical telescopes: three in
Bulgaria, two each in India and Serbia, and one each in Greece, Georgia, and
Latvia. We monitored the blazar quasi-simultaneously for 16 nights in the V and
R bands and 8 nights in the V, R, I bands and examined the light curves (LCs)
for intra-day flux and colour variations using two powerful tests: the
power-enhanced F-test and the nested ANOVA test. The source was found to be
significantly (> 99%) variable in 4 nights out of 27 in R-band, 1 out of 16 in
V-band, and 1 out of 6 nights in I-band. No temporal variations in the colours
were observed on IDV timescale. During the course of these observations the
total variation in R-band was 0.89 mag observed. We also investigated the
spectral energy distribution (SED) using B, V, R, and I band data. We found
optical spectral indices in the range of 0.878+-0.029 to 1.106+-0.065 by
fitting a power law to these SEDs of PG 1553+113. We found that the source
follows a bluer-when-brighter trend on IDV timescales. We discuss possible
physical causes of the observed spectral variability.
