To select the number of colors in an image, use the X Colors command to decrease the color depth of an image and reduce the file size. If you specify 16 or fewer colors, the image is saved as a 4-bit image. If you specify 17 to 256 colors, the image is saved as an 8-bit image. For some file formats, particularly GIF, file compression is sensitive to the number of colors in the image. For example, using 100 colors instead of 256 colors produces a smaller file size (and produces faster download times), even though both files are 8-bit images.
To decrease the color depth to a selected number of colors:
To emphasize the importance of certain colors in the conversion, select an area of your image (optional).
Choose Image > Decrease Color Depth > X Colors (4/8-bit).
In the Number of colors edit box, enter the number of colors (from 2 to 256) for the converted image’s palette.
In the Palette group box, select the palette options. See About Palette Options.
In the Reduction method group box, choose a color reduction method. See About Color Reduction Methods.
In the Options group box, mark or clear these check boxes:
Boost marked colors by If you selected an area of your image and you are using the Optimized Median Cut or Optimized Octree methods, mark this check box to make the colors in the selection more important by a factor from 1 to 10. The selected colors will stand out from the rest of the image.
Include Windows’ colors Mark this check box if you want the 16 standard Windows colors included in the converted image’s palette.
Reduce color bleeding If you chose the Error Diffusion reduction method, you can mark this check box to make the left-to-right bleed effect of colors less noticeable.
Click OK.