I suspect that black paint was the least expensive paint available post war for some time, particularly in the austerity years immediately after World War II. At the time I am also pretty sure a high gloss black paint scheme with gold pin stripping was considered the epitome of class and quality. You also can't call a Black Shadow a Black Shadow without the engine being black. But let's be honest. A Rapide in red with gold pin striping and polished engine cases is a thing of beauty.
I know Vincent manufactured a few Rapides and Comets in Red. But they should've pushed the option more. That is pure rolling art.