Some relevant WR livery and crest info from E.R.Mountford's book on Caerphilly Works:
On 2 February 1948 instructions were issued that GWR lettering on locos was to be replaced by BRITISH RAILWAYS in full. Also letter "W" was to be painted below the cast numberplate.
On 12 March 1948 instruction for painting the "W" was cancelled.
Late 1949 the first BR crest (the cycling lion) was introduced in two sizes. The first engine to receive the smaller version at Caerphilly was
5682 on 18 October 1949, and the first with the larger size were 4255 and 5649 on 8 December 1949.On 10 January 1950 instruction that GWR practice of painting the shed code on front hanging plate was to be discontinued, and replaced by cast shedplates on smokebox door, with painted details on inside of cab roof.
In November 1956 an instruction issued that certain main line passenger engines were to be painted green with full lining out, whilst some smaller and mixed traffic engines were to be painted green but unlined.
On 25 January 1957 the instruction was amended to ALL green engines being fully lined out. The first engine so treated at Caerphilly was 4152 on 15 February 1957
The second BR crest was first applied at Caerphilly to 5680 on 19 March
1957.An instruction in July 1957 stopped the practice of lining tender fenders.
6386 on 29 July was the last to have two panel tender lining.From March 1958 some smaller engines were turned out black instead of lined green as previously. 2266 on 28 March was first engine turned out in black instead of lined green.
The practice at Caerphilly (only) of painting reversing rods of tender engines red was stopped by an instruction from Swindon in November 1958.
An instruction in March 1962 stated that only engines having Heavy General repairs were to be repainted, and only main line passenger classes were to be lined green.
My recollection is that the first style BR crest was still reasonably common in 1962.
Hope this helps.
David