I remember that it had become a standard on PC's and it was called High Color, or something? Neither CGX or P96 support 18bit modes. You go straight from 16 to 24bit modes.
There actually was a "fake" 18-bit mode used in quite a lot of PC-demos in the mid-90's. This was basically achieved by setting the palette to include 64 shades of both red, green and blue, and the drawing the RGB-components on different lines, which with a high enough vertical resolution kind of blended into 18-bit colors. Probably best illustrated by this image:
http://www.oldskool.org/demos/explained/explained/htmlpictures/18bitori.gifThis was ofcourse not a hardware graphicsmode, but a programmed one.
