intervals are short.. there's a delay between drop in temp and plant actually getting "colder" which shortens any potential length of time where the plant gets colder than the average ... so i doubt this is an issue.
if you consistently get fall colors late in bloom and can rule out gentics due to near 100% consistency, then you can probably assume it got a little too cold at night, but still might not be a major issue relative to 67F... borderline temp (at worst) and only a temporary state as mentioned above.