How can I create a plugin that changes the title color of a website?

The exact way to do this may vary based on the theme you are using, but here’s a simple plugin that hooks into the wp_head action hook and adds some style to the header:

<?php
/**
 * @package PACKAGE_NAME
 */
/*
Plugin Name: Plugin name
Plugin URI: https://plugin-website
Description: Plugin Description
Version: 1.0.0
Author: Plugin Author
Author URI: https://author-website
License: Plugin License
Text Domain: text-domain
*/

add_action( 'wp_head', 'wpse321903_add_styles' );

function wpse321903_add_styles(){ ?>
    <style>
        .page-title {
            color: white;
        }
    </style><?php
}