Thought I was on NCD for a second

Disclaimer, not a lawyer, etc…

In abstract, no. “Sailing under false colors” is considered a legally permissible act of deception under the laws of war, so long as the vessel clearly identifies itself as a military vessel belonging to its proper nation of origin prior to opening fire.

In practice, obviously this is a bit of the law that doesn’t have a clear answer in modern, beyond-line-of-sight warfare. It worked reasonably through World War 2 (where, indeed, several nations used disguised warships for various purposes - my favorite being the time the British disguised an explosives-laden destroyer as a German warship, then rammed it into a drydock). But what would that look like in a modern scenario? Lifting a flag, even if no one can see it? A radio broadcast?

There’s also the reverse side of it: That, if a nation is using warships disguised as civilian vessels, anyone fighting them would have wider defensible grounds to more aggressively engage any suspicious or uncertain vessels. This would, undoubtedly, eventually cause civilian casualties - but such is the risk (and sometimes, intent) of using a civilian disguise.