Bristol radial engines......cowling colour

Most technical artists render the leading edge of wartime Bristol
radial engine cowlings a copper-bronze colour.

Several wartime colour photographs (of the Vickers Wellington etc)
show this same effect.

The 'Halifax' restoration/reproduction at the Yorkshire Air Museum has
it's Hercules engine cowlings painted a reddish-brown colour.

This seems to be characteristic of most Bristol radial engine
installations of the period.

Being involved in the restoration of a 1945 vintage 'Hercules' engined
aircraft I am greatly interested to know how this effect came about.

It is NOT RUST because the original cowlings are made of Aluminium.

Was it due to a special formulation/colour of the original protective
paint? If so can anyone refer me to the specification?

Or was it as a result of heat on standard night black camouflage paint.
The cowling is separated by an air gap from the Townsend exhaust
collector ring, but substantial heat would build up particularly during
idling/taxying. All the same the colour seems so uniform on photographs
that I doubt that a heat effect would create it.

Even if a heat effect is the case.....I assume that YAM have painted
theirs because their restored engines cannot be run to create a natural
effect.

Any directions gratefully acknowledged.

RC