Colour illustrations are almost non-existent in brochures from the 1920s and early 1930s. Where they exist, it is due to the art department glamming them up a bit.
However, the answer is BLACK on the cylinder head, block and dynamo/starter. Spray or brushing tins of suitable black enamel are readily available.
For exhaust manifolds you can use motorcycle POT BLACK although not on the aluminium water branches, pulleys, carburettors, etc. which stay bare.
The only colour item found on all models between 1922 and 1939 are red fan blades; a safety feature to help avoid unwary knuckles being rapped. The red is simply POST OFFICE RED.