If you look on the 'net, the shade of maroon/red C15's seems to vary (slightly) all over the shop.
Your best bet is to find an unfaded paint example - inside a toolbox, or under the tank, or behind the kneepads,
or inside the brake backing plate - and have it matched at your local paint shop.
Failing that, you may get lucky and have someone who has been through this tell you what they matched it to.
But even paint shops will tell you to paint everything from the one can, the next one may differ slightly in shade.
And it could have been the same even from BSA...