In the latest entry to his blog, Mikko Koppanen shows how to take the power of Imagick and harness it to check and see what sorts of colors an image contains:

The code in the example reduces the image colors to 10, then discards all but one pixel of every color and then creates the palettes out of those colors. This might not be the most accurate way to do this, but at least it's fast.

His code makes three different "palettes" from the image - average, dark and light - and outputs them as a range of colored boxes for each all of the three palettes. Examples of the output are also included in the post.