Nameplate colour is black, locos with red backgrounds as built were 60002, 60007-60017, 030. However when you get to late BR days it's all but clear some black plates went red then back to black on a whim it seems, some were light blue.
All the colour photos I have of Merlin show a black bias, but she was always allocated north of the border and light blue plates seem to come from that region more than any other I found. Example 60011 was originally red plates but by 1960 was black, then in March 63 was light blue but by May 63 was black again! For Merlin I'd go with black
Plaque, well I've no direct colour photos but Google turns up these two.
The plaque which Merlin is based on is H.M.S Merlin, a landbase for the RN which shows light green?
http://thumbs2.ebaystatic.com/d/l225/m/m9aWq-KI1FQUtmKqrk56ufA.jpg
However Richard green thinks it might be light blue?
http://www.locos-in-profile.co.uk/Prints/LNER/A4/Merlin.html
Having said that, Plymouth Military gifts, who seem to be pretty authentic
http://www.plymouthmilitarygifts.co.uk/store/rn_crest_fridge_magnets_m.html
Have it as green
I would think that being as it's a recognised military crest then LNER would follow the correct heraldry colours, but, who knows!
The real loco crests must exist somewhere, but even then there is no proof that the extant colour is original, as can be seen from nameplate colours noted above.
A little more Flickr finds these shots where it does look blue?
https://www.flickr.com/photos/irishswissernie/8620465786/sizes/h/
https://www.flickr.com/photos/articdriver/6291839400/sizes/o/
What I find odd, is that Merlin is Haymarkets top hitter on the Elizabethian and there's more photos of that train and service being the top express on the ER at the time, yet 60027 seems very camera shy, certainly in colour.
) got a colour picture of the crest carried by Merlin. Also a photo of it in post war garter blue with skirts removed as no27. Need to know what colour the nameplate is. 
