PluginColorExtract is a Rainmeter plugin that will process an image and output colors suitable for meters in the same skin based on that image. It's intended to be an approximation of the algorithm iTunes uses to color album views, and as such is oriented around square images. (In fact, it will internally work off a copy of the image resized to 50 pixels square, so images that have a different aspect ratio may lose some information in the processing.)
To write a skin using this plugin, please consult the documentation for available options. Builds for 32- and 64-bit Rainmeter, as well as an example skin, are available under the releases. To install the plugin without installing the example skin, place the DLL corresponding to the version of Rainmeter you have installed in the Plugins directory, as described in the documentation.
Shoutouts to Seth Thompson in this Stack Overflow answer for sharing his implementation of the algorithm, and @arcanis for writing colibri.js, an implementation of the algorithm I could actually read and understand. This couldn't have happened without them.