We report observations of colour in the inner coma of Comet C/2013 UQ4 (Catalina) with the broad-band B and R filters. We find significant temporal variations of the colour slope, ranging from -12.67 ± 8.16% per 0.1 μm to 35.09 ± 11.70% per 0.1 μm. It is significant that the comet changes colour from red to blue over a period of only 2 d. Such dispersion cannot be characterized with an average colour slope. We also observe Comet C/2013 UQ4 (Catalina) in infrared using Spitzer and find no significant CO/CO2 gaseous species in its coma. Therefore, we classify Comet C/2013 UQ4 (Catalina) as a dust-rich comet and attribute the measured colour slope to its dust. We analyse the colour slope using the model of agglomerated debris particles and conclude that the C/2013 UQ4 coma was chemically heterogeneous, consisting of at least two components. The first component producing the bluest colour is consistent with Mg-rich silicates. There are three different options for the second component producing the reddest colour. This colour is consistent with either Mg-Fe silicates, kerogen type II or organic matter processed with a low dose of ultraviolet radiation.
