You are correct. MacX 1.5, supports up to 8 bit color graphic images (256 colors). The X11 R5 server protocols can support 24 bit color graphic images, however, Apple's implementation of MacX 1.5, an X11 R5 based server, still operates at 8 bit graphic mode because some of the graphic code was from MacX 1.2.
Article Change History:
08 Apr 1996 - Updated article to MacX 1.5.