There's a pretty famous thought experiment about this: https://en.wikipedia.org/wiki/Knowledge_argument

The discussion portion of the research paper (linked here) summarizes what many of the patients felt after treatment:

Treated patients with achromatopsia did not open their eyes after the surgery with a feeling of a miracle, i.e., the different shades of gray were not replaced by a range of rainbow colors. Nevertheless, color detection was evident in all patients’ treated eyes. Long wavelength (red) was the most detectable, but even for this stimulus, detection was slow and did not reach the level of saliency that evokes a pop-out effect.

When the adult patients were asked to describe how they perceive the red stimulus in the treated eye (when presented in a way that allows them to detect it), they often admitted that they had no appropriate words to describe it. When encouraged to find the exact wording, they said it glows differently, shines, or appears on a different plane than the background.

Probably not what most expect but still interesting regardless.