Atari 800
The early Atari 400 and 800 computers use a palette of 128 colours (a bit similar to the one used on the Atari 2600 console, or the Commodore Plus4), using 4 bits for chrominance, and 3 for luminance. Screen modes may vary from 320×192 to 40×24, using 2 or 4 simultaneous colours, or 80×192 using 16 colours. After 2 years (late 1981) the CTIA graphics chip was replaced with the GTIA chip thus increasing the palette to 256 colours (in new modes). (CTIA and GTIA)
The Atari 8 bit line of computers (400, 800, XL and XE models) have Display List Interrupts (DLIs) which permit many more colours on the screen at once. An example is the game Alternate Reality:The City (link to image)
Read more about this topic: List Of 8-bit Computer Hardware Palettes