This gives them excellent night vision, but their vision during the day or whenever they are in the presence of light is quite poor. They have to wear blindfolds, essentially, to block out the light as much as possible.
I have itched to write a character with this condition ever since I first read its description in Oliver Sacks' book, The Island of the Colorblind.
But I'm asking myself, 'Does Garedin need it? Does the plot need it?'
I wanted Garedin to have the ability because it would make him better able to see the shipwreck in chapter 1 or 2. But you know, ships have wrecked during night-time storms for millennia, and normally-sighted people have always somehow managed to effect rewcues. So I'm asking myself if perhaps the achromatopsia is just A Cool Thing I Want to Use. Logically, if I can tell the story without it, then I shouldn't use it.
I guess I"m just annoyed that, if I ditch the colorblindness, I lose the chance to explore how a royal court would operate when the royal family are achromats, but most of the courtiers are not. I'm presuming that there would be some intermarriage between the royal family and other noble families, so some courtiers might well achromats, also. I like exploring the cultural aspect. For me, that is almost as much fun as writing the book, itself.
I also have to ask myself if, as a person who is legally blind, am I just projecting a variant of my own eye problem into the fictional world, and is that lame or valid?
I suppose the answer is to just write the story the way I feel it, and let the chips fall where they may.