Strip "color" id/value from querystring and use as body class. Prepends "theme-prefix-" to the color name. Example: For http://www.example.com/?color=blue the class would be "theme-prefix-blue."

string-body-class.php

<?php //remove opening tag

add_filter('body_class', 'string_body_class');
function string_body_class( $classes ) {
	if ( isset( $_GET['color'] ) ) :
		$classes[] = 'theme-prefix-' . sanitize_html_class( $_GET['color'] );
	endif;

	return $classes;
}