I have found that event colors were reset when there are multiple DAVdroid accounts (at least one with event colors enabled, and at least one event colors disabled) because of some unexpected/insecure Android API behavior (sync adapters may modify raw events of other sync adapters, which seems to be not intended and most likely a [security] bug).

Please tell us whether it works with the APK I have sent to you by email, where this problem is worked around.