The flags or banners which distinguish the ships of different nations. Also, the regimental flags of the army. Hauling down colours in token of submission, and the use of signals, are mentioned by Plutarch in Themistocles.
To sail under false colours and chase is an allowable stratagem of war, but firing under them is not...
The Sailor's Word-Book
A rite practised in the army, but not in the navy. ...
A mistress, or concubine; water colours being, like their engagements, easily effaced, or dissolved....
Dictionary of The Vulgar Tongue by Francis Grose