Image colors palette

[Medium]

What images? There are those, that come with an associated palette, and others that have none at all. From the latter you may derive a palette, but then the question is, what restrictions you need/want to apply to it (color space, number of colors, etc.), and if it is okay to thereby reduce the image's color depth (i.e. make a 256-color paletted image from a 24-bit true color one). Then, maybe even dithering comes into play, and more so palette optmizations, which are actually not that trivial if done well. Or do you not need to actually change the image itself but simply obtain a "most common colors and shades"-table?

So. What exactly do you want/need to achieve? What is your readily available base data, and what do you need to output?