A copunctal point is where color confusion lines
intersect for a certain dichromat. It corresponds to the missing cone primary.
The red, green and blue dots are the copunctal points of protanope (missing long
wavelength cone), deuteranope (missing middle wavelength cone) and tritanope (missing
short wavelength cone) respectively.
Click a chart to redraw it if it is distorted
Here the copunctal points derived by Smith and Pokorny (1975) are used because they are probably
the only set of copunctal points for 1931 CIE color matching functions.
Please see Stockman, A., Sharpe, L.T. (1999) for a thorough review
of cone primaries. Colblindor has a very readable blog explaining copunctal points and confusion lines.