Would sailing under British colours have made Bismarck a pirate?

Would disguising Bismarck as a British battleship with British colours have counted as piracy?

Sailing under British colours would not itself count as piracy in this account, provided the ship did not attack under that flag.

Sailing under another flag is distinguished here from attacking while displaying it. The proposed Bismarck deception would depend on changing colours before opening fire. Discovery would still expose the German ships to interception and battle. The proposal remains a risky hypothetical, with its legal distinction limited to the rule stated in this exchange.