Plugin CSS not enqueing

If you want to add inline CSS, use wp_add_inline_style().

<?php
/**
 * Plugin Name: WPSE 257470
 * Description: WordPress StackExchange question 257470
 * Plugin URI: https://wordpress.stackexchange.com/questions/257470/
 * Author: Nathan Johnson
 * Licence: GPL2+
 * Licence URI: https://www.gnu.org/licenses/gpl-2.0.en.html
 */

//* Don't access this file directly
defined( 'ABSPATH' ) or die();

if( ! class_exists( 'wpse_257470' ) ):
  class wpse_257470 {
    public function plugins_loaded() {
      add_action( 'wp_enqueue_scripts', [ $this, 'wp_enqueue_scripts' ] );
    }

    public function wp_enqueue_scripts() {
      //* It's necessary to have a 'base' CSS file that we can attach to
      wp_enqueue_style( 'wpse-257470', plugin_dir_url( __FILE__ ) . 'style.css' );
      $color="#257470";
      $custom_css = ".custom_color { color: $color };";
      wp_add_inline_style( 'wpse-257470', $custom_css );
    }
  }
  add_action( 'plugins_loaded', [ new wpse_257470(), 'plugins_loaded' ] );
endif;