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I know it's possible to export to 256 colours and to change colour mode to 256 colours, flattening the image. But I'd like to be able to setup the image to work only in 256 like you can set it up to work in RGB or HEX. Pretty sure this used to be possible, but can't figure out how to do it in PS5.


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To convert a document to Indexed color mode, use Image > Mode > Indexed Color. You can select from a variety of choices (web, Windows OS, Mac OS, etc.) as a starting point. As Horatio points out, this immediately disables about 95% of Photoshop's capabilities: you forfeit layers, filters, channels, adjustment layers and masks, and you have only a limited selection of destructive adjustments available.

If you don't mind working within these limitations, fair enough, but I would strongly recommend a different, non-destructive and far more flexible approach.

Work with your document in RGB mode, but keep a 256-color Posterize adjustment layer at the top of the stack. This leaves all of the regular tools of Photoshop at your disposal, but still gives you an immediate and accurate view of your final output.

When you're ready, save your PSD then convert to Indexed Color and save separately.

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256 color RGB is essentially an indexed color swatch set, where each pixel is tagged with an index number and then the software looks up the number and finds the color value to use.

So to mimic that,you can set up the color palette with the only the color swatches you want, and make all color selections from that. Many of the filters and effects will not be usable if you want to keep to those specific colors. Anti-aliasing etc probably is out too.

Blending and transparency are out, because they will create colors which are not in your palette.

This is not strictly doing what you want, but if you are careful, when you export to indexed color (gif etc), the result will probably be nearly exact.

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