I pared addons down to nUI and Leatrix GFX. Here are cropped screenshots, showing the parts of the UI that are bugged by 16x16 color as enabled by Leatrix GFX. In addition to all life and power bars, the castbars and action bars are blacked out (though not the icons on the action bars).
I run windowed. Most Xorg window managers have severe problems alt-tabbing in and out of WoW when it's in full screen mode. nUI RL returns the textures and (I am guessing) 24x24 color, as opening the Leatrix GFX GUI and re-checking 16 bit color returns the blacked-out textures.
Leatrix GFX is available here on WoW Interface, if you wish to try to reproduce the bug:
http://www.wowinterface.com/download...eatrixGfx.html