> and only 16-bit color (4 bits per pixel).

The "high color" (16 bit) mode was 5:6:5 bits per channel, so 16 bits per pixel.

> So 153,600 bytes for the frame buffer.

And so you're looking at 614.4 KB (600 KiB) instead.

"Windows 3.1 primarily used palette-based color modes, common modes included 16 colors (VGA/EGA) and 256 colors (SuperVGA)"

Right, so 16 color, not 16 bit color.

To be frank, I wasn't aware such a mode was a thing, but it makes sense.

I recently installed NT4 (including Plus!) in an emulator with a VESA video driver, and was greatly surprised when about half of the icons that I thought of as “Windows 2000” (including the memorable “My Computer” one with the bulbous sky-blue screen) turned out to be available even there, provided a non-indexed mode. The rest were the more familliar 16-color-compatible 95/NT4 ones, making for an incongruous result overall. I guess what I want to say is that 16-color compatibility is a large part of the 95/NT4 look from which 2000 very carefully departed.

640 * 480 / 2 = 150KB for a classic 16-color VGA screen.