Sorry, but 216 is the limit (at least for now). The scheme that we used was based on a defacto-standard established by an organization that helped webpage designers select colors that would make it easy for people to understand the information and graphics presented on a computer display.
At the time that we implemented the current 216 color palette, NX/UG was only supporting 16 colors so the move to 216 was considered to be both significant and more than adequate. And before anyone asks why 216, the scheme that this web design consulting company used to represent their palette of colors was a '6 x 6 x 6 cube' where the eight primary, secondary and tertiary colors, that is Red, Green, Blue, Cyan, Magenta, Yellow, Black and White, were placed at the corners of the cube and the spots between them were then assigned evenly graduating colors. Note over the years, while the basic scheme was maintained, we have tweaked the colors a bit based on feedback from our users as to what would constitute a better palette for the types of models and images that our customers were creating. And while you are limited to 216 different colors, you have full capability of editing any of them to meet what might be your special needs.
Of course, when it comes to photo-realistic rendered images, NX uses a totally different scheme based primarily on material assignments thus allowing a virtual infinite 'palette' of colors. However, for normal wireframe and shaded images, the limit is 216 colors.