Battleship grey.

For a week now there has been an impressive array of navy warships plying up and down the Clyde   ( prompting some of the more cynical wags amongst us Scots to observe  "Thought the British Empire had given up sending a gunboat to troublesome territories to cow the restless natives" :-D )
And this passing ship in its fetching shade of blue /grey prompted me to hot foot to the decorators shop to buy  something like it to refresh the wall.
Confronted with 20,000 different shade cards ,picked what I thought was a delicate soft calm grey that wouldn't go amiss on a battleship!
Skipped home and set to painting the wall :-)

Whereupon the soft grey I had chosen metamorphosed into an iridescent turquoise blue that a peacock would give its eye teeth for !! :-0

Why does it do that ???   :-(