You can see all of the colors that used in an image. "Analysis->Color Palette" menu shows color palette of active image. As you can see in Fig 8-3, this window has some ability to show you witch color is used in which place and how many pixels have similar color.
If you want to sort color, you should right click on palette and choose your method for sort or you can save palette as excel file or an image (Fig 8-4).
NOTICE: Saving palette as excel file is time-consuming and may take time several minutes.
Figure 8‑3 . Color Palette Window
Figure 8‑4. Right Click on Palette