Hello James,
I am not sure that I can help to tie the paints down with any exactitude, but I can offer some observations.
At the time of the North Cape action in Dec 42, Onslow would have been in the MS series paints. The A1 G and B series paints were first promulgated in AFO 2106/43 in May 1943. The MS series that existed from early 1941, or thereabouts, was matt (as were 507A and 507C at this time, per CAFO 290/1941, issued Feb 1941) and the matt paints were found to be of inadequate durability. The A1 G and B series that replaced the MS series had linseed oil added to the paints to increase their durability (and so were slightly glossy).
AFO 2106/43 and CB3098/43 together tell us that A1 B30 as a practical match for B6 in colour and tone, and G20 as a practical match for MS3 in colour and tone. They also explain that G10 and 507A are exactly the same colour and tone, as are G45 and 507C (again, the finish is slightly different).
Prior to her repairs, Onslow was in a what appears to be a three-paint Special Emergency Fleet Scheme, as you have shown. She emerged from those repairs in what appears to be a two-paint scheme of the same style (it looks like a simple over-paint of the two darker tones into one tone). The following links help to confirm this, the first being Onslow on return from North Cape, the second being after the ensuing repairs:
http://www.iwm.org.uk/collections/item/object/205147517http://www.iwm.org.uk/collections/item/object/205120251(That image from worldnavalships.com throws me a bit: as nice as it is, I think that the contrast is out to a considerable degree, so other than its role in indicating that there were three paints, I will avoid being distracted by it for this discussion.)
(The Polish ship modelling community takes quite an interest in the O and P class destroyers, as you may know. There is a 1/200 paper model of Onslow from Modelik that makes it look as if they intend to show her in MS1, B6 and Admiralty white, as does Profile Morskie on their website. What they have based that on, I don't know, and I would view it with some hesitation, but here is the model if you are interested
http://i-am-modelist.com/2012/05/28/hms-onslow/ )
Alan Raven's Warship Perspectives RN series covers the SEFS in Volumes 2 and 3, but not this scheme specifically. He does use the wrong nomenclature for the paints prior to AFO 2106/43's release, but we can see what he is portraying, and that is a range of schemes using G10/507A, G20/MS3, B30/B6, G45/507C, and white. His sources are never directly quoted for each proposed scheme, so again I approach the information with hesitation, but that does help to narrow the likely options. His advice is also backed to some degree by Hodges, however, in RN Warship Camouflage 1939-1945 on pp 70 and 71. He shows Barfluer and Tartar in late SEFS variants, the paints used being G20, B30, G45 and white.
I think that you could therefore assume with some level of comfort that at the North Cape action, the lightest paint was white, as you have suggested.
The mid-range paint seems too dark in contrast to the lightest paint to be 507C, so I suggest that it is B6.
The darkest paint could be either MS3 or 507A. To state what is probably obvious, the purpose of these schemes was to balance the desired invisibility to U boats with the need for visibility to the fleet. The compromise made was for the bows to be light to hide the vessel (in the manner of the WA schemes) when attacking head-on, but darkly contrasting at the sides to make them more visible to the fleet and so avoid collisions whilst station-keeping. With that in mind, I have trouble seeing the bows as being 507A - the contrast with white would seem too great to help hide them from head-on. It would seem more 'dazzle' than 'disruptive'. My gut feel is that it is G20/MS3.
Based on all of that, if I was to take a punt (and without documentary evidence, it can never be more than that), I would offer MS3, B6 and white as the scheme.
Regards,
Lindsay