Hi Dale,
Could be either, I suspect it depends on the period and the locomotives you are modelling, in the book Working Steam, Stanier Jubilees it clearly shows both e.g. 45596 & 45562 in 1966 red name and smoke box number plates, 45636 red nameplate 1960, 45632 red nameplate 1963, the majority however appear to be black, on page 79 there are four photos of nameplates 3 are black one is red which appears to confirm this. I don't know if it is still in print but it has some great shots of the Jubilees almost all in colour, well worth getting if you are interested in the class.
Rgds.....Mike