I only propose this as an idea. From photos I have seen around the web it does seem that the camoflage is very much darker than the standard RAF Med scheme. I do know that orders were given to repaint maltas spits in a grey/blue colour so It may have been possible that FAA colours and USN colours were used.
My answer would be speculative based on what I’ve read, along with some straight forward thinking.
Temporary application … if you mean it was to be scrubbed off once on Malta, then not likely. True though on some applications it seems hastely done, sometimes squadron codes were covered and replaced with a single letter while on other examples the markings were left as is and roughly painted around without masks.
I think pilots welcomed this “new” scheme and similar colouring was applied to aircraft (when time allowed) already situated on the island. This blue or grey scheme was just as important to the defenders flying off Malta as it would be better to meet the enemy over water before they had reached their targets on the island proper.
The first delivery, via Wasp, was painted in standard desert scheme of Dark Earth/Middle Stone over Azure Blue; while on the carrier, they were repainted by the ship’s personnel. Nobody can say, for certain, what colour it was, but a (now deceased) Wasp crewmember said that no paint, from the U.K., was taken on board, which means that the favoured colour (in some quarters) is the blue used for painting the ship’s superstructure, known as Deck Blue 20-B (colour is available from White ensign Models.
After the first delivery, and before Wasp’s second, Malta Command asked that all future deliveries should be in Temperate Sea Scheme, which was Extra Dark Sea Grey/Slate Grey. Although the scheme normally called for Sky undersides, Malta had already rejected it as too pale, so it seems logical that only the top colours were overpainted, leaving Azure, or Light Mediterranean, Blue underneath. There is a (sort of) corroborative colour photo, of Keith Park making the inaugural flight from a new airfield, possibly American-built on Gozo, and it’s clearly in Temperate Sea over Azure Blue.
As a small extra, Spitfires delivered to Malta were tropical-type, with the large under-chin intake, because these carried a larger oil tank, which was needed for the long journey from carrier to Malta.