Gfx tiles saved in Arcade Game roms
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i'm trying to see how are the tiles saved doing some reverse engineering using old roms gfx files (this example trying with snow bros arcade set one) . 

This is what i know now :

- Each color point to a palette which can change (then when you change palette you change that color in tiles).

- Tiles have a 16x16 size.

I'm a little confused cause when i see the file in an hex editor . I don't know how many bytes are a pixel or how are they order to see the tile.

Any idea of how this works? . Thanks for your time.