I also have built the St Nectan and had trouble getting the correct "Brown" paint. They don't make brown cars these days so the choice of spray paint quite is limited.
In the end, I mixed up my own shades from brown and white and yellow (believe it or not !) until it looked reasonable to the eye. Even then, I changed my mind during the build.
Sorry I can't be more specific, but it's what suits you and what you'll be happy with.
Cheers
Ken