Showing custom post user wise with different color in wordpress

You can add the post ID in the user meta, which is like a space in database to save contents related to an user and a key (the name of the field you’re saving). Then, each time you show that post, you can check if the user meta contains the ID of it, and you can change the color.

Once the user subscribes to the post do

add_user_meta( $user_id, 'post_subscribe', $post_id );

And when you show the post

$user_posts = get_user_meta( $user_id, 'post_subscribe' );
if ( in_array( get_the_ID(), $user_posts ) ){
    // Do something
}

PD: To know data about the user logged in (like the ID), you can use wp_get_current_user().

$current_user = wp_get_current_user();
$user_id = $current_user->ID;