Bitmap Export Dialog Box - Color Depth
Color Depth
You can create bitmaps with the following number of colors:
32bit alpha channel
CMYK (TIFF files only)
24 bit Millions of colors
8 bit 256 colors
4 bit 16 colors
1 bit 2 colors
Many bitmap formats support only some of these color options. For example, the GIF format supports 256, 16 or 2 colors. Other options are dimmed.