Show your true colours
"To reveal your real character or intentions, especially after hiding them."
"Colours" is an old nautical term for a ship's national flag. Ships, privateers and naval vessels especially, would sometimes fly a false flag to approach a target without alarming it, only hoisting their real, or "true," colours at the last moment before attacking. The trick was such a well-known piece of naval deception that it produced a general-purpose idiom for someone who has been concealing their real intentions.
Sources: Documented naval practice from the 17th–18th centuries; cited in maritime history and the Oxford English Dictionary's entry for 'colour, n.'
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