Clockwise will override any existing event colors that you created through Google. However, Clockwise will only change an event's color if that event is in a category that has a color assigned to it. For example, if you manually color an event that is in the Ad-Hoc category blue, that event will stay blue if you do not assign a color to Ad-Hoc events.
If you remove the color-coding, all the colors will be reset to the calendar default. The calendar will not revert to the previous manual color-coding.
If you have any questions or feedback, please email us at firstname.lastname@example.org.