XRGB

The Apple III and, I believe, the Apple II with the Apple II RGB card, supported the AppleColor Monitor 100, which was an XRGB card.

XRGB is a digtal interface with 4 bits, providing high quality 16 color.

The 4 bits are decoded to a 16 color palette by the monitor.

This is the monitor: https://www.apple.asimov.net/documentation/hardware/video/AppleColor%20Monitor%20100%20User%27s%20Manual.pdf

some text about it: https://retrocomputing.stackexchange.com/questions/21688/what-is-an-xrgb-video-signal-not-the-modern-xrgb-mini-din

What's not clear is the Apple III video memory map.