On a hunch, I took the plunge and -- knocking on plenty of wood -- upgraded my display driver (directly from windows updates via the control panel rather than a web browser -- this gave me an even newer version than the DELL site would have let me download). Even though the description for a similar upgrade which I had seen on the web never mentioned OpenGL with so much as a word, it seems that the OpenGL API was either installed or updated by this driver upgrade, because now I can run the OpenGL version of the game.
Result: when I switch to "Draw All" the "Graphic Effects" in the OpenGL version, the colors in the game change. Not only do upgraded creatures now look different from the basics, but people and objects also have different colors now. E.g. different chitin armors now have different colors. Wow! It's like going from a black and white world to one rendered in color.
So, yes, the DX version of Geneforge 5 is seriously crippled (and Windows installation disks don't install OpenGL, you need to update your display driver for that, sheesh). That was the problem. (sadly I have to live with some sluggishness now, since the game only displays colors properly (i.e. meaningfully, so you can tell different things apart!) on the very highest graphics setting -- a truly odd design choice by Spiderweb Software, sort of as if only the deluxe model of a car got breaks installed)
Problem solved ... kind of.