The TSeng ET4000 W32 versions with enough memory support at least 24-Bit color so far as I remember, I think some of the last Cirrus and even Trident cards support higher color modes. But performance goes down the shitter very quickly once you start turning these things up though due to the limits of the ISA BUS, you might be able to slightly negate this by ramping the clock / divisor up but then you start risking stability. The high color modes are usually good enough for standard Windows usage with everything stock anyway, its only if you start scrolling large objects over the screen or trying to play videos that it really dives, at least most of the time, I have heard other people having less luck with these things.

Almost any "newer" ISA card can do 15/16-bit. Of course I'd recommend only the ones with Windows GUI acceleration - if you plan on using Windows 😉

For example:
- Cirrus Logic 5422 (does Win 3.1 but not Win95). Newer models are fine (5424, 5426, 5428, 5429, 5434)
- S3 801, 924...
- Tseng Labs ET4000/w32
- WDC 90C31
- ATI Mach32, Mach64

Fastest all around is CL-5434 but fastest in Windows is by far ATI Mach64 as it rivals even better VLB cards.

No ET4000 variant has an integrated RAMDAC, so it depends on the RAMDAC of the card if it can do hicolor or truecolor.
Seconding the WD90C31.

Thanks. I'd like to get one and plug it into a Tandy 1000 tl which has eight bit slots and see if I can write a 320x200 15bpp game demo. Then I'll do the same in my Tandy 1000 RSX which is a 386sx 25. I'd also like to take this list and determine which can run in a 8 bit slot.