I used to run an art social network 15 years ago that did this automatically with every piece of art uploaded and then it let you search for art by color that way.

Basically

$average = new Imagick( $file );

$average->quantizeImage( $numColors, Imagick::COLORSPACE_RGB, 0, false, false ); //Reduce the amount of colors to 10

$average->uniqueImageColors(); //Only save one pixel of each color