Are hairless cats really hairless?

No, so-called "hairless cats" are not truly hairless because various parts of their bodies maybe nude but the remainder e.g. the muzzle and the feet (often) are covered by fine down, fuzzy hair. Sometimes hairlessness progresses with age. There is a fine covering in the young adult cat which gradually disappears leaving a bare, wrinkled skin over much of the body of the full-grown adults.

Bicolor Sphynx. The fine down hair strands in the hind legs are
pigmented I believe, which is why there is a dark patch and the tail
 is dark too. This is melanin in the down hair strands.

There is a bit of discussion about whether hairless cats can have a pattern on their skin. You do see 'bicolour' Sphynx cats with the two colours being the skin colour and the other colour being an inky darker colour. 

I had thought that the darker coloration was due to the pigment producing cells, the melanocytes, colouring the skin but I now believe that this is due to fine down hairs having the coloration that the cat's genetics gives it. The melanin in the hair strands being produced by the melanocytes.

To be clear, if you see a bicolour Sphynx or any other hairless cat what you are seeing is hair strand coloration. The point is that you can barely see the hairs because they are very fine and short, being the down or undercoat.