Comment by andrewla

It seems that this would be well-suited to a simple online test -- show a square with one color and a square inside of that with a different color, and ask the user whether the inner square is brighter (or too close to call). Aggregate this across users and assess the fit to the CEILAB or other color spaces. It seems like you could get almost all hn users to take a stab at this for a bit before they get sick of it.