In which colour the engine was originally painted ?
Was the jaguar logo and 3 1/2 liter logo painted in a different colour?
See factory colour illustration:
I don’t think the Jaguar logo was highlighted in a another colour.
Peter
Yes, black for all cast iron and steel parts, but aluminum parts are unpainted. Logos were not painted separately.
The tappet cover on the left side is an interesting point. Early engines such as my '38 had black cast iron covers, where on later engines such as my Mark V it was unpainted cast aluminum.
The inside of the block and gearbox case was red, but this was a special paint called Glyptal, which was to seal in the microscopic sand particles remaining in the surface layer of the iron casting after the casting was shot blasted at the foundry. It prevents the sand from working out and getting into the bearings. Differential cases were also dipped in Glyptal but it is usually orange. This was on the rough casting before any machining work, so it is not seen on machined surfaces. The sand boundary layer was cut away by the machine tools.
Occasionally one sees black painted carb dashpots, but I don’t know if anybody has worked out a time frame for those.
Those of you who have seen my '38 saloon on ssjaguardata.com will please note that I disavow any knowledge of the pastel green paint on my engine. 
The black suction chamber covers on the carbs were a motor show thing, RR/Bentley had black carbs and I suppose it was a bit of “me too” by Jaguar.
I seen the same on a Motor Show Daimler which also had terminal blocks painted gloss black.