Rhamnus frangula (Alder Buckthorn)
I have only found two of these up here in our area, but apparently there are lots of them down in Boulder. Native to Eurasia, this plant lives happily as a volunteer in North america. In some states it is considered very noxious.
This is an "Ah Hah!" moment! Suddenly something anomalous stands out in the woods.
The bright autumn leaves really make this little plant visible.
The flowers on this shrub are very small.
Under the microscope, the tiny flower reveals its colors.