The eventColor_index value is set after modify by Digical. Informant sets only eventColor and displayColor, the former looks like the only needed to set a specific color for the event.
CalDAV only supports indexed colors. According to https://developer.android.com/reference/android/provider/CalendarContract.EventsColumns.html#EVENT_COLOR, EVENT_COLOR should only by updated by sync adapters and not regular apps like Informant. Instead, EVENT_COLOR_KEY should be used, which then causes EVENT_COLOR to be updated automatically and DAVdroid to recognize the color.
Can you please report that to Informant support and provide some kind of reference here?
It’s COLOR, but for VTODO, not only VEVENT. Do you handle this while syncing tasks (VTODO) or is it only for calendar (VEVENT) ?
As far as I know, OpenTasks does not support per-task colors.