I looked in Roy Bacons book on BSA Twins Restoration, and this is what it says under the appendix on colours.
'A7 1947 - 50
All painted parts either black or Devon Red, this including frame, forks, oil tank, battery carrier, chainguard and mudguards.
Petrol tank chrome plated with black panels lined in gold or Devon Red panels gold lined.
Wheel rims chrome plated with either black or Devon Red centres gold lined.
Chrome plated headlight rim, handlebars, tank cap, brake back plates, battery strap, fork oil seal holders and exhaust system.'
Since you have painted the rest of your bike black you may want to keep the tank panels black with gold lining.
I don't have a modern equivalent colour to suggest for Devon Red, but I am sure either someone else on the forum can help. Alternatively you can contact one of the classic motor cycle paint specialists.
I hope this is helpful.