I have some images to back up the drawings I made for the barrels. The basis is one image from a series of three of HMS Prince of Wales at anchor showing the outer ST barrel of A-turret elevated. There is one good top view without much perspective deformation; all barrels appear equally long per turret in each image, so ideally suited to measure the length and position of contrast changes. I think we can assume a less than 5% relative error of the colour position on the barrels.
POW_15.jpg
Even without knowing what colours are used on the turret roofs, this image tells us that the darkest colour is
not present on B-turret barrels and at least three tones can be distinguished on A-turret.
POW_16.jpg
We get the same information for Q-turret; at least three tones, even though we cannot be certain from this shot what goes where (From the same image, so same contrast).
POW_17.jpg
However, from this shot we get most answers. If we assume that the barrels were also painted in no more than 5 tones, knowing that the darkest tone is not present on B-turret, and seeing the contrast change between upper and lower barrel halves close to the glacis plate we can conclude that B & C are on the barrel upper halves and D & E on the lower halves. This is one of the few images showing the C/D contrast; it's impossible to tell even for the rest of the same barrel in the same shot. From this shot I made and worked from the assumption that A/B/C are dorsal and D/E ventral colours.
POW_18.jpg
This shot is quite interesting: note the one-but-closest barrel to the camera (barrel once removed sounds a bit odd?). Anyway, there is one contrast change just in front of the diameter change from light to dark and then there's a second contrast change from dark to darker. I conclude that C must have been applied to the ventral side.
POW_19.jpg
The farthest barrel barrel seems to show a band of C to the ventral side as well.
Genreally speaking, it appears that dorsal A and B are usually teamed with ventral D & E respectively (plus a lot of deviations). The rest follows from an analysis of all frames we have of HMS POW at Critical Past and British Path�. I have about 100 Mb of individual frames saved to disk from both sites: a bit too much to post!