the be of structure.

zyc50

It's a bit stuffy language and reflects the use of unnecessary words

they are of the same color = they are the same color
it is of utmost importance to = it is utmost important to
it is of (his) best interest = it is (his)best interest

It happens a lot and with other verb structures as well:

be abusive of (abuse)
be applicable to (apply to)
be benefited by (benefit from)
be derived from (derive from)
be desirous of (desire or want)
be determinative of (determine)
be in agreement (agree)
be in attendance (attend)
be indicative of (indicate)
be in error (err)
be in existence (exist)
be influential on (influence)
be in possession of (possess)
be in receipt of (have received)
be in violation of (violate)
be operative (operate)
be productive of (produce)
be promotive of (promote)
be supportive of (support)
The simple verb (in parentheses) is usually better.