As already said, the brochures tell the story - apart from the 20M3 bikes, where it seems to be a bit sketchy...
From '69 onwards it seems fairly straightforward and covered by the brochures, but this is my take on the '68 bikes (all fastbacks with round tank badges):
The very first bikes seemed to be plain silver tank & panels with no decals on the panels. Orange seats possibly also a feature?
Some very early bikes in candy green & candy red over plain silver tanks. One green one had matching side panels. These were delivered to the customer already painted, but I wondered if these were done by the importer and not the factory. I believe there was customer pushback over the dull paint schemes which the factory later responded to.
It seems they were followed by (or in parallel with?) metalflake silver which was in the gelcoat on the fibreglass, retaining plain silver painted side panels.
2-tone (Metalflake silver panels with candy red or more rarely sapphire blue) bikes were definitely done by the factory. I've seen them referred to as 'export colours' as UK bikes were predominantly gelcoat green or red, which also appeared after the early batches.
Post N-V round tank badges:
Not sure if any candy painted bikes were sold in the UK, but the 2-tone paint scheme continued into the non-round badge late '68'-'69 bikes (still 20M3) and had black with gold lining tank decals. Plain silver side panels still featured, later with the 750 Commando logo. I have no idea when this was first introduced; the US Cycle Sport magazine test bike had no side panel graphics, but did have '750 Commando Made in England' graphics either side of the tail fairing, which also had a Norton logo in the middle immediately behind the seat. It made reference to also being available in '3 solid colors; Red, Green & Silver'.
As far as I can tell, the Type 'R' was only sold with a candy red over metalflake silver gelcoat tank.