WordNet records no synonym and no antonym for ink. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
Synonyms, antonyms and the broader and narrower relations for ink come from Princeton University’s WordNet. Full sources and licences.
a liquid used for printing or writing or drawing
dark protective fluid ejected into the water by cuttlefish and other cephalopods