'Colours' are simply the names that we assign to different wavelengths of light (or to mixtures thereof). So they're not 'discoverd; they're simply assigned names. Paint manufacturers, for example, keep assigning all sorts of new names to the products in their ranges but they've not really been 'discovered', simply given names.
If you recognise 'black' as a 'colour' though (rather than as the absence of colour), you could say that vantablack was discovered in the early years of this century. It's strictly a product name but the word can also used to describe its colour (or lack of it), since it's the blackest black in the world - or, rather, it was until last year when an even blacker substance was discovered (but neither that material, nor its colour, seem to have been named yet).
https://en.wikipedia.org/wiki/Vantablack
https://news.mit.edu/2019/blackest-black-material-cnt-0913