c. Geranium red, sky blue, etc. are manufacturers' names for certain
colours
and are not consistent.
No, the actual colours of geraniums, the sky, and buttercups are reckoned to
be the perfect paint primaries, but there are no corresponding pigments in
the real world.

d. All colours are available in emulsion, as is black.
Compare it to an oil base black and you will see that it is, at best, grey.
Practically all blacks tend towards blue, which is why ICI used to charge
the earth for a perfect black dye. If the perfect primaries existed, you
could mix them in equal parts and get black