I found out somewhere else online that the colour for the air filter cover and other parts mentioned above may be as follows: (Whilst he doesn't specifically speak of Datsun 1200's its probable that it may be the same colour used)

"The colour is still listed on the 'British Standard' database as B.S. 18C39, with HMG ( Her Majesty's Government ) Glidden number GL31557, and Glidden number name 'Dolphin Blue'."

With respect and gratitude to "gav240z"
If you follow the quote you should be able to find the source post. (not sure if I can link directly to it)
I hope it helps anyone here.
I'm still going to get a few samples and compare it all and judge for myself but its hard to say because I don't remember what it looked like when I was a kid to compare exactly but I feel a little bit better knowing that someone else has also done some research here so they may be spot on right.

I feel like the rocker/valve cover colour is going to be much harder to match though.