The SNES uses indexed graphics. Typical (Mode 1) output would be 3 background planes and 1 sprite plane stacked on top of eachother. Background planes would display 32x28 tiles (NTSC) where each tile is 8x8 pixels. Each tile would use one palette out of a total of 8 and every palette consists of 15 colors + transparency. There are 8 palettes for backgrounds and 8 for sprites. Background 1 and 2 are 4BPP and background 3 is 2BPP. Bitdepth means that every pixel had 4 bits (or two) to assign a color, so 15 + transparency.
There are other screen modes, zero to (the famous) 7, but mode 1 was the most common afaik.
This playlist is really cool if anyone wants to learn more :)
https://www.youtube.com/playlist?list=PLHQ0utQyFw5KCcj1ljIhExH_lvGwfn6GV