So I was sat here manually picking the colors from an image and it was taking ages. Turns out there is a really easy way to do this in Photoshop CS4. It exists in earlier versions but the process is a bit different and I couldn’t actually find anywhere detailing how to do it with CS4. So here’s my version for posterities sake.

• Open your image as normal.
• Go to Image->Mode and change it to Indexed Color.
• Click “OK” on the “Flatten visible layers” confirmation. We’re not going to save the image, we’re just extracting the colors.

• On the “Indexed Color” popup:

  • Set the Palette to “Local (Selective)”
  • Colors to 256 (or a number above what you reckon the maximum really is for your image)
  • Forced to None
  • Tick Transparency
  • Dither to None
  • Tap OK to convert the image.
• You’re image should still look the same as it did before although we’ve converted it to the Indexed Color mode.
• Now go to Image->Mode and at the bottom you should now see the “Color Table” option as selectable so click that.
• Tada! You should now see all the colors from your image as a big ol’ set of color swatches. Now you can do Save to create an ACT or PAL file which you can then load into your main swatches panel or other ACT/PAL compatible application.
• Now you just have to remember to not save your image file 
• We’re done with the image file. Remember that if you save it now you will lose all your layers and blending options. This doesn’t matter if it was a flat image to start with but it’s definitely worth keeping in mind.