Official blazon
Gules a Cross raguly couped proper the arms joined in fylfot between two Ducal Coronets in chief Or the bottom part of the cross enfiled with a Ducal Coronet of the last beneath each coronet a Nail of the first each piercing the cross.
Origin/meaning
These are not registered with the College of arms but are obviously very ancient.
The coat of arms first appeared on the Royal Charter granted by Henry V in 1413. The red represents the blood of Christ; the green cross, the living cross of Saint Helena; the three crowns, the three Kings from the Nativity Story; and the nails holding the three crowns, the nails which held Christ to the cross.
St. Helena was traditionally born in Colchester in the 3rd Century AD. In the early fourth century she made a pilgrimage to the Holy Land and discovered the true cross growing as a tree at Golgotha. Later she is said to have discovered relics of the Magi, now in Cologne (Köln) Cathedral. Three crowns also appear in the arms of Cologne.
The arms were altered at the time of reformation so as not to appear idolatrous. The living cross was changed to two white ragged staves and the nails were removed. These arms were confirmed in 1558 and recorded in use in 1634. The town council reverted to the original arms in 1915.