The article mentions that they've barely scratched the surface (as it were) of understanding how the optic nerve works in a large way (part of that is color encoding).
I would assume they've gotten this far on a little guesswork and labwork testing how the optic nerve receives and transmits impulses to the brain in a very simple way. So far, it looks like they've figured out how it encodes light/dark shapes (thus grayscale), how the optic nerve encodes the input it receives from the retina as color and how the brain receives that encoding to process it is still a mystery.
I would assume they've gotten this far on a little guesswork and labwork testing how the optic nerve receives and transmits impulses to the brain in a very simple way. So far, it looks like they've figured out how it encodes light/dark shapes (thus grayscale), how the optic nerve encodes the input it receives from the retina as color and how the brain receives that encoding to process it is still a mystery.