I recently acquired a s/h oil tank for C15 / B40. The tank is the late model (1965 onward) tyre with the froth tower. I noticed that on the rear side it appears to the legendary Sapphire Blue. Apparently there is some doubt whether any bikes were supplied in this colour - so I guess this oil tank indicates that at least one was! The 'Sapphire Blue' appears to have been sprayed over a white primer. (The rest of the tank has been sprayed black during the last 60 years.. I might strip it back carefully to see if there are any more signs of the blue.)
I got the oil tank to paint red... which was the original colour of my bike. But now I am hesitating...
Pics may not give a true representation but it looks like the standard non-flamboyant Nutley Blue?