To be honest, nobody knows.
Oxford blue was always the colour supplied by the depot, but nobody now knows who supplied it, or what shade it actually was - There are very few colour photos of early boats, and chances are they'll have faded anyway.
In the 1880's, the blue used was 'French Ultramarine', but when that changed, gawd only knows.
I've scoured the Lifeboat Journal archive, and there's nothing in there at all.
Pick a colour that looks OK, and use it - nobody can ever prove it's the wrong colour, as nobody knows what colour it was in the first place!
(As Andy has said, we like the look the look of Ford Balliol blue - which, of course, is just their trade name for Oxford Blue - which another car manufacturer had trade marked).
Lance