The Motorola 6845 CRTC chip is quite versatile, and one of its unique characteristics is that it knows and cares nothing about the resolution or number of colors on the screen. It is just a display address generator, which is meant to provide some external hardware with a memory address that contains data to be displayed at some part of the screen. What to do with this address and data is completely up to the computer hardware, which can interpret it whichever it wants. So there is nothing in the 6845 chip that prevents using it to display 256, 4096 or 16777216 colors on the screen.