It's a satin black or semi-flat/slightly off semi-gloss sheen black. If you want an exact color code for concours exact, then IIRC there is a paint code/type/color in the factory shop manual somewhere - I'm pretty sure that the tank is the same satin black as used on the engine tins, heater flapper boxes, J-tubes, etc. around the 914 in black.
AA & maybe some other sources sell a CD manual in pdf format for $30-something if you don't have the 8 binders, & those have some extra "bonus" mauals included, in addition to a scanned 8 volume set.
Tom
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