color-classes.css

.has-black-background-color {
     background-color: #2a2a2a;
 }
.has-black-color {
     background-color: #2a2a2a;
}
.has-gray-background-color {
     background-color: #7a7a7a;
}
.has-gray-color {
     color: #7a7a7a;
}

editor-color-palette.php

<?php
/**
 * Add support for custom color palettes in Gutenberg.
 */
function tabor_gutenberg_color_palette() {
	add_theme_support(
		'editor-color-palette', array(
			array(
				'name'  => esc_html__( 'Black', '@@textdomain' ),
				'slug' => 'black',
				'color' => '#2a2a2a',
			),
			array(
				'name'  => esc_html__( 'Gray', '@@textdomain' ),
				'slug' => 'gray',
				'color' => '#727477',
			)
		)
	);
}
add_action( 'after_setup_theme', 'tabor_gutenberg_color_palette' );

old-editor-color-palette.php

<?php
// Adds support for editor color palette.
add_theme_support( 'editor-color-palette',
	'#2a2a2a',
	'#7a7a7a',
 );

line 5 of color-classes.css should be color: … instead of background-color: …