Brother MFC-3420C - Manual (Page 171)

USING YOUR MACHINE WITH A MACINTOSH ® 9 - 16
Color Type
Black & White—Set the image Type to Black and White for text or
line art. For photographic images, set the image type to Gray (Error
diffusion) or True Gray.
Gray (Error Diffusion)—is used for originals with photographic
images or graphics. (Error Diffusion is a method for creating
simulated gray images by placing black dots instead of the true gray
dots in a specific matrix instead of true gray dots.)
True Gray—is used for originals with photographic images or
graphics. This mode is more exact because it uses up to 256 shades
of gray. It requires the most memory and takes the longest transfer
time.
Colors—Choose either 8-bit Color, which scans up to 8-bit Color or
24-bit color which scans up to 16.8 million colors. Although using
24-bit color creates an image with the most accurate colors, the
image file will be about three times larger than a file created with 8-bit
Color.