You could just create different style sheets for each color profile, and load those depending on which month it is. Here’s a basic example showing how you could load a halloween.css file for october, and an xmas.css file for december. The code below would go in your themes functions.php file:
add_action('wp_enqueue_scripts', 'seasonalCSS');
function seasonalCSS()
{
// register stylesheets to be loaded safely
wp_register_style('halloween', get_stylesheet_directory_uri() . "/halloween.css");
wp_register_style('xmas', get_stylesheet_directory_uri() . "/xmas.css");
switch(date('n'))
{
case 10: // if it is october...
wp_enqueue_style('halloween'); // load halloween.css
break;
case 12: // if it is december...
wp_enqueue_style('xmas'); // load xmas.css
break;
}
}