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.

ordered dither
ordered dither. The parameter should be one of
  threshold        Threshold Non-Dither
  checks           Checkerboard Dither
  o2x2             Ordered 2x2 Diffused Pixel
  o3x3             Ordered 4x4 Diffused Pixel
  o4x4             Ordered 4x4 Diffused Pixel
  o8x8             Ordered 8x8 Diffused Pixel
  h4x4a            Halftone 4x4 (angled)
  h6x6a            Halftone 6x6 (angled)
  h8x8a            Halftone 8x8 (angled)
  h4x4o            Halftone 4x4 (othogonal)
  h6x6o            Halftone 6x6 (othogonal)
  h8x8o            Halftone 8x8 (othogonal)
  h16x16o          Halftone 16x16 (othogonal)
  hlines6x4        Horizontal Lines 6x4

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.