Adding variable in WordPress shortcode

Rather than passing the variable contents in to the shortcode, you would probably be better to pass in the URL parameter you are setting, i.e. [post color="color" width="980" heigh="610"]. Because your shortcode will execute during load in PHP, you can then tell your shortcode function to store the value of $_GET['color'] to later use in that function.

You might do this like:

function build_the_thing( $atts ) {
    $pageColorURL = $_GET[$atts['color']];
    return $pageColorURL;
}
add_shortcode( 'post', 'build_the_thing' );