To reduce the file size of images with few colors, decrease the color depth of an image to 16 colors (4-bit). This color depth is especially useful for simple graphics on Web pages that you want to load quickly.
Note: When you decrease color depth to 16 colors, the resulting image has an image palette that contains 16 specific colors. You can change any of those colors (and create interesting effects) by editing the image palette. See Working with Image Palettes.
To decrease the color depth to 16 colors:
To emphasize the importance of certain colors in the conversion, select an area of your image (optional).
Choose Image > Decrease Color Depth > 16 Colors (4-bit).
In the Palette group box, choose a palette option. See About Palette Options.
In the Reduction Method group box, choose a color reduction method. See About Color Reduction Methods.
Note: The Ordered Dither option is available only for the Windows color palette.
In the Options group box, mark or clear these check boxes:
Boost marked colors by If you selected an area of your image, you can mark this check box to make the colors in the selection more important by a factor of the value you enter. The selected colors will stand out from the rest of the image.
Reduce color bleeding If you chose the Optimized Median Cut or the Optimized Octree palette and 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.
Understanding Color and Color Models
How Monitor and Print Colors Differ
Making a Palette Color Transparent