It's great to learn about different aspects of First World War aviation, and I have often wondered about Nivo and other night fighting colours. There isn't a lot written about it relatively speaking, although you mentioned the Windsock Datafile Fabric Special Number 2, which I don't think I have ever seen, so there are obviously publications in circulation which address night fighting camouflage. I had an idea it was a dark slate green. what does the statement "and had no relation to the matt qualities of varnish" mean? I have been told that matting agents had not been invented during the First World War, and every authentic fabric sample I have ever seen confirms this. Varnish and dopes tended to be applied very sparingly onto aeroplanes, so the gloss or semi gloss is not always as obvious as it would be if it had been a full, thick coat or a full, thick series of coats, and it is obvious when examining authentic samples and photographs that varnish and dopes became dull relatively quickly.
Unfortunately I can't help you with the Methuen or FS colours. I matched some samples of MvR's triplane 425 / '17 to the Methuen Handbook of Colour once but I used Colin Ower's book, which I had borrowed. As you know, it is reasonably difficult to find. I have or had a Pantone reference colour fan, but I don't know where it is unfortunately.
I haven't seen the Pantone book, my SE5a and Sopwith Pup drawings, or my DH9 notes for quite some time .... must be getting old and senile ....
I usually don't lose ANYTHING!!! Hopefully they will all show up. Hopefully one day I will be able to find and to afford a Methuen handbook of colour, but in the meantime I can't help you unfortunately, other than to tell you that there are various cross references to the various colour systems which have been mentioned on this site, and there would be a lot of information available online. Someone will post some colour references on this thread soon. I have seen various colour chips posted on various threads on this site.
Regards,
David.