Here are descriptions of the palette choices in the Indexed Color window.
By selecting a palette, you're deciding how the 16 million colors in your photograph will be reduced to 256 or less.
If your photograph already contains less than 256 colors, this choice will appear.
Select Exact if you don't want to reduce the existing colors.
Select System (Mac OS) or System (Windows) if your photograph will appear on only one of these operating systems.
Select Web when your photograph will only be appearing on a website.
The colors in your photograph are reduced to the 216 colors recognized by most browsers.
When you select Uniform, the colors of your of your photograph are evenly reduced to 216 colors.
This palette doesn't take the frequency of the colors in your photograph into account.
This palette reduces the colors in your photograph based on their frequency.
For example, if your image contains mostly skin tones, the adaptive color palette will be mostly skin tones.
Use Local (Adaptive) for single photographs.
Use Master (Adaptive) if you're working on multiple open photographs that have the same color palette.
This palette is weighted toward reducing the colors in your photograph to those to which we are the most sensitive.
Use the Local (Perceptual) for a single photograph.
Use the Master (Perceptual) if you're working on multiple open photographs that have the same color palette.
The Selective palette will reduce the colors in your photograph to the web-safe colors.
Use the Local (Selective) for a single photograph.
Use the Master (Selective) if you're working on multiple open photographs that have the same color palette.
Use this choice if you want to create your own custom color palette.