@culi The OKLrCH is interesting. It seems to give the least amount of banding when cycling through hues, at least in my eyes. I would think it would be really good for measuring distance. Well, with one transformation. I would use the hue (H) as a polar angle in cylindrical coordinates and C as the radius. Then convert to cartesian and measure distance. I don't know how I would actually implement this, but it's an idea
... I guess color space is like a double cone with extra pointy parts. I wonder how different this double cone would be from the RGB cube on its diagonal.