thinking of more recently published material relating to the Sowerby factory, then it's always possible that Glen and Stephen Thistlewood might be able to assist in some way. These authors produced a three CD ROM disc set which details much factory catalogue information, and although I can't recall there being any specific reference to colours, they might be able to point you in a useful direction.
Regret I don't know the source of Glen and Stephen's information - it might have been the archives already mentioned by Anne, or another source, but perhaps worth an email to ask.
Until a year or two back Glen did maintain contact with the GMB, but I've not seen the lady posting for a couple of years or thereabouts - their contact details should still be on the Board somewhere.
P.S. don't forget that there is some quite detailed info. in Ray Slack's book regarding the factory's formulae for colours and chemical make-up of some of the well known things like Pearline and Queen's Ivory Ware etc.