Effects > Artistic > Posterize
Posterize
The Posterize filter allows you to reduces the number of colors in the image.
1. Choose Effects > Artistic > Posterize
2. Use the following options:
 Levels
This option allows you to limit the color palette or range of tones. Values can range from 1 color to 255 colors.

Min = 1, Max = 255, Default = 180.
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