Thank you for the complement; the HMS PoW scheme was a joint effort by me and Dick. I also read 507B was a 50/50 mix but that does should not result in a blue tone. If I compare 507B to B5 using the WEM tins knowing that a) WEM is a near-perfect match for the Snyder & Short color charts and b) WEM tins have the exact color of the contents on the lid, you'll see these interpretations are nearly identical
Also, from my spreadsheet with all the colour data (
link) I have it follows there is only one ship listed having both B5 and AP507B: HMS London. So, one could state the colours rarely if ever (assuming HMS London indeed carried both) appeared in the same scheme and B5 is in fact the replacement. This doesn't necessarily means they are the same tones as there are more combinations of colours that do not or rarely appear side by side, however, this does make sense if one assumes that AP507A/B/C were replaced by MS2/B5/MS4. I updated the spreadsheet with this assumption in the second half. White does not appear much with other colours, simply because it wasn't used often, but B6 still never shows up with MS3 and M4 other than allegedly in HMS PoW and Euryalus.
I also do no mind more confusion and I'd love to find more material indicating B6 was a blue and slightly darter variant of MS4/507C. It would make the comparison between what Raven states (B6 indeed applied to HMS PoW) and the footage (both B&W and colour) more agreeable. Does the recipe for B6 appear anywhere? One wonders if there is a student out there with in interest chromahistory...