How does the allocated bits per colour pixel affect the battery life? It’s the backlighting of the LCD that consumes a majority of the power. Are you referring to the graphics processor?
The MacBooks screen buffer may use shared memory in the main RAM rather than separate memory. And the screen buffer probably stores the data as 24 or 32 bits per pixel anyway, and the screen circuitry just ignores the two least-significant-bits per sub-pixel. Six-bit subpixels cannot be packed in memory very efficiently, so the screen buffer might as well use a full byte for each subpixel, especially since memory is so cheap nowadays.