8 bit computer refers to the width of the bus, so a c64 had 8 channels to transmit data/instructions and an upper limit of the memory it could have before banking.

8 bit graphics is where 256 colours would come in, in terms of total palette to choose from, but not necessarily display without chasing the beam.

That’s why 256 colours moving on screen at once was rare even on 16 bit computers until the vga standard :)