Before the days of computerised colour standards a Chief Engineer of the Southern Railway was on a visit to the Isle of Wight with his Works Managers. The group stopped outside an Opticians while the great man studied a lorgnette chord on display in the window. He went inside and purchased it. Emerging, he snipped it into two inch lengths and handed them out. "That is the colour I'm talking about gentlemen. That is the colour I want our carriages to be". Thus Ashford, Eastleigh, and Worthing had a unified data base to work from.
FWIW, here is the colour palette of the supposedly exact colours defined for RAF aircraft in WWII (from ISBN 0853682712 Aviation Colours of WWII) :-
I guess the colour in question is Azure Blue?