Hi Magnus,
Thanks, it has been interesting. I've looked at numerous period color photos of B-25s and their R-2600, including NARA photos of the NA assembly plant. I found that some had each cylinder blackened (none glossy) but the images from the NA assembly line show the cylinder barrels black, and the cast heads are steel. The nose casing (not installed in the photos) are always grey, (some with a notably bluish tint), with the governor atop glossy black. The plug wires are a grayish-brown.