First define the preferred number of colors in your image. The actual number of colors in the image may be less than your request, but never more. Note, this is a color reduction option. Images with less unique colors than specified with this option will have any duplicate or unused colors removed. Refer to quantize for more details.

You also have a choice of several options that affect your image colormap. Choose from these options:

apply Floyd/Steinberg error diffusion to the image. The basic strategy of dithering is to trade intensity resolution for spatial resolution by averaging the intensities of several neighboring pixels. Images which suffer from severe contouring when reducing colors can be improved with this option.
black & white
transform the image to black and white.
global colormap
create a single colormap for a sequence of images.
netscape color cube
map image to web safe colors.
map to clipboard
map image to clipboard colors.
gray image
transform the image to gray colors.
segment an image by analyzing the histograms of the color components and identifying units that are homogeneous with the fuzzy c-means technique.
preserve transparent pixels
by default color reduction does not preserve and transparent pixels in your image. Choose this option to reduce the number of colors in your image while preserving any transparent pixels.