Tim,
Looking at the BS 381C charts, "Maroon" (541) actually appears as a shade of brown on the computer screen. However, the book on the Mk1 coaches states that BS 381 540 (Crimson) was the shade specified for the 1949 "blood & custard" livery and this was replaced by the all-over "maroon", using BS 381 541 as the reference, with the intention of replicating the LMS colour.
Ralph may be able to throw some light on this, since he has an old tin of LMS "lake" which resembles nothing like the colour seen on LMS coaches, until they had been varnished.
Regards,
John.
P.S. On re-reading, I should make it clear that the LMS 1946 livery is the maroon colour mentioned and not the pre-war crimson "lake". It may be that the same "lake" was used but with a different finishing varnish.