250's were green background with white numbers.
350's were blue background with white numbers.
AHMRA, VRRA, FIM et al generally use(d) the same colours.
There are some beautiful photos of the '60's race bikes on the web and an outstanding youtube video of a Yamaha TR2 (350) and TD2 (250) in a museum setting.
If you're being pedantic, Stan Nicholson from the VRRA probably has the correct colour for the green but it looks pretty much like Sherwood or British Racing Green to me.
For my ersatz 350 TR2, I lucked out and found a Tremclad spraybomb blue that pinned the colour I needed "close enough for me".
Sounds like a cool idea. Post a picture when you're done.
BTW, should be oval, not rectangle number plate for that era.
Hope this helps.
Vintage Motorcycle Fiberglass