It's all really a matter for speculation. No one knows for sure what color that blue was. Most seem to think it was from US Navy stocks since they were on the Wasp.
I had a long discussion about it with Spitfire historianPeter Arnold and Clive Denney who runs the Historic Aircraft Company at Legends before they did the Blue painted Spit V for the Merlins over Malta event. If Peter and Clive didn't know for sure, no one does.