The following table contains all unique (green) RGB-255 color values the Neo-Geo is able to display. There is a quite big gap between color #31 (RGB 117) and color #32 (RGB 136)
but on the real system (NG CD) it does not look that big. It seems that the MAME resistor values are not 100% correct.

HPMAN, if you want, you can download a demo CD with this graphic and test it by yourself: http://www.neohomebrew.com/forum-pics/color-test-cd.rar