Asters are part of the Composite family Asteraceae. Their flower heads are yellow-green to red, and the rays range from white to lavender or purple. Once thought to be related to the old world Aster genus, we now know they are different, and have been split into at least three other genera.
It is often noted that Calico can be identified by the red flower centers, but this is not reliable. Early on they are all white. Most if not all the smaller asters will have the flower head turn reddish with time. Size and number of rays are what to look for.