Union Monitor Colours
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The colours used for the Union monitors varies with the area to which they were assigned.
Those assigned to the blockade of Charleston and taking part in the attack on fort Sumter were painted black, possibly having a coloured ring around the stack to identify which was which.
Those assigned to the James river were black, with vertical surfaces (turret, stack etc) painted white. They also had a coloured identification ring around the turret at the height of the gunports, to identify each vessel.
Other monitors appear to have been left as bare metal (which would soon rust) or painted grey, particularly those operating on the rivers.