Full color evolution of AT 2000ch in g − r (orange), g − i (green), and r − i (purple). Previously published color values by Wagner et al. (2004), Pastorello et al. (2010), and Pastorello et al. (2013) (converted to the Sloan photometric system) are shown as well as our photometric results from the PTF/ZTF survey observations. We only show the previously published data points for those color indices where we have added new observations. The vertical blue dotted lines indicate the dates of the observations that we collected in the radio range and the arrows mark the times of known (black) and newly detected or suspected (gray) eruptive episodes based on Eq. (1) with their 1σ (3σ) uncertainty indicated by the gray (light gray) shaded areas.