There were Unix machines with real DirectColor graphics hardware
As I mentioned yesterday, one of the questions I wound up with was whether there ever was any Unix graphics hardware that actually used X11's unusual DirectColor mode. Contrary to what you might expect from its name, DirectColor is an indirect color mode, but one where the the red, green, and blue parts of a pixel's colour value index separate color maps.
The short version of the answer is yes. Based on picking through the X11R6 source code, there were at least two different makes of Unix machines that had hardware support for DirectColor visuals. The first is (some) Apple hardware that ran A/UX. Quoting from xc/programs/Xserver/hw/MacII/README:
These Apple X11 drivers support 1, 8, and 24 bit deep screens on the Macintosh hardware running A/UX. Multiple screens of any size and both depths are accommodated. The 24 bit default visual is DirectColor, and there is significant color flash when shifting between TrueColor and DirectColor visuals in 24 bit mode.
Based on a casual perusal of Wikipedia, it appears that some Quadra and Centris series models supported 24-bit colour and thus DirectColor.
(Support for DirectColor on Apple A/UX appears to have also been in X11R5, released in September of 1991, but the README wasn't there so I can't be sure.)
24 PLANE SUPPORT FOR HCRX24 AND CRX24
This Xhp X11 sample server supports two modes for the HCRX24 and CRX24 display hardware: 8 plane and 24 plane, with 8 plane being the default. [...]
In depth 24 mode, the default visual type is DirectColor.
This support seems to have appeared first in X11R6, released in June of 1994. HP probably added it to HP/UX's version of the X server before then, of course.
It's possible that some other Unix workstations had graphics hardware
that directly supported DirectColor, but if so they didn't document
it as clearly as these two cases and I can't pick it out from the
various uses of
DirectColor in the X11R6 server source code.
(Since X11R6 dates from 1996 and PCs were starting to get used for Unix by that point, this includes various sorts of PC graphics hardware that X11R6 had drivers for.)
There seems to be support for emulating DirectColor visuals on other classes of underlying graphics hardware, and some things seem to invoke it. I don't know enough about X11 programming to understand the server code involved; it's several layers removed from what little I do know.
I admit that I was hoping that looking at the X server code could give me more definitive answers than it turned out to, but that's life in a large code base. It's possible that there's later Unix graphics hardware supports DirectColor, but my patience for picking through X11 releases is limited (although I did quickly peek at X11R6.4, from 1998, and didn't spot anything). People with more energy than me can pick through the x.org history page and the old releases archive to do their own investigation.
(The intel driver manpage suggests that the i810 and i815 integrated Intel graphics chipsets had hardware support for DirectColor, but that this support was removed in the i830M and onward. I would assume Intel decided it wasn't used enough to justify chipset support.)
PS: Note that later releases of X11 start dropping support for some older hardware; for example, the 'macII' support disappeared in X11R6.1. For what it's worth, the release notes up through X11R6 don't mention removing support for any graphics hardware; however, I haven't checked through all X11 releases from X11R2 through X11R4 to see if DirectColor hardware appeared briefly in one of them and then disappeared before X11R5.