Dirk, the 'Sea Grey Medium' I presume is 'Medium Sea Grey', which is the colour required for the underside of the Typhoon, and all RAF day fighters finished in the Temperate Scheme, that is, ETO, from late 1941. The Slate grey can be used for FAA aircraft, and is also a handy colour to have for mixing various 'weathered' shades of US Olive Drab. So, best to keep 'em both!