The only times when quiet buttons have color by default is when it's an action we want to emphasize. For example.. "Thank," or when going through a certain workflow, highlighting the next progressive action like "Next", especially when it's destructive like "Overwrite edit". These examples don't exist currently because we don't have a "Thank" action on Flow or do we use quiet buttons for progressive actions within workflows. But when we do, they will be colored. All other instances will be black.  

Hope that made sense. Hope everyone had a great start to the year!