Although both cruisers appear similar, Cornwall had her forward (P1 and S1) Twin 4 inch guns positioned ahead of her elevated 8 barrel pom poms and just behind her bridge. A good photo illustrating the forward location of Cornwall's twin 4 inch gun mount may be found on page 14 of Man O' War 1 County class cruisers.
On HMS Berwick, the forward twin 4 inch mounts are located almost in line with the forward funnel which places them under the rear half the elevated platforms of the 8 barrel pom poms. A good aerial photo illustrating Berwick's twin 4inch gun positions can be found on page 19 of the same book.
Cornwall recommissioned in 1938 in what appears to be Home Fleet colours of dark grey overall with the upper masts painted light grey. Neutrality patrol stripes were then painted on to B and X turrets. Cornwall appears never to have been fitted with radar and still had a diamond shape DF on her forward mast at time of loss.
Photo's I have for Cornwall at Simonstown in 41 and later at Durban in late 1941 show no 20 mm guns or even lewis guns added on up to that point in time. At some point either in 1940 or 41, Cornwall's after mast was reduced in height with the upper mast section removed. The forward upper mast remained painted in light grey up to her loss.
Overall, Cornwall in the 1941 photo's I have appears to be painted in a medium tone grey with the four vertical corners of her hanger structure painted a very dark or black colour and the hanger door painted in a disruptive camouflage pattern that still can be seen painted on at time of loss. The late 1941 Durban photo's show Cornwall's wood decks to be unpainted at that time.
The artwork on the Trumpeter box of Cornwall is wrong in showing her in light grey while under attack, but is correct in showing her lower port holes as being plated over.
Berwick appears to have recommissioned in 1938 to 39 in a light grey overall scheme with the steel sections of her deck painted a dark grey. Berwick had several disruptive early wartime camouflage schemes and is listed by Alan Raven and John Roberts (British Cruisers of World War two page 427.) to have received (5) 20mm single guns and types 281 and 284 radar in 1941.
Both cruisers had two large carley floats mounted on port and starboard sides of the hanger structure when recommissioned. By 1941, only Cornwall still had these floats mounted on the sides of the hanger.
Photo's of Cornwall taken at Simonstown and Durban in 41 show one large and one small float additionally mounted vertically on main deck level on either side of the after mounted twin 4 inch gun on the port side only.
I hope others will add on to this thread.