To reduce the number of colors used in a 24-bit image, decrease the color depth of an image to 32K (32,000) or 64K (64,000) colors. Displaying images with 32K or 64K colors on older monitors results in better refresh rates than displaying 24-bit images with 16 million colors.
To decrease the color depth to 32K or 64K colors:
Choose Image > Decrease Color Depth and then choose 32K Colors or 64K Colors.
In the Reduction method group box, select the method. See About Color Reduction Methods.
Click OK.
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Making a Palette Color Transparent