Index: src/wp-content/themes/twentyfifteen/inc/customizer.php
===================================================================
--- src/wp-content/themes/twentyfifteen/inc/customizer.php	(revision 30242)
+++ src/wp-content/themes/twentyfifteen/inc/customizer.php	(working copy)
@@ -42,8 +42,9 @@
 	) );
 
 	$wp_customize->add_control( new WP_Customize_Color_Control( $wp_customize, 'sidebar_textcolor', array(
-		'label'   => esc_html__( 'Header & Sidebar Text Color', 'twentyfifteen' ),
-		'section' => 'colors',
+		'label'       => esc_html__( 'Header & Sidebar Text Color', 'twentyfifteen' ),
+		'description' => esc_html__( 'May only be applied to the sidebar on wider screens.', 'twentyfifteen' ),
+		'section'     => 'colors',
 	) ) );
 
 	// Remove the core header textcolor control, as it shares the sidebar text color.
@@ -56,8 +57,9 @@
 	) );
 
 	$wp_customize->add_control( new WP_Customize_Color_Control( $wp_customize, 'header_background_color', array(
-		'label'   => esc_html__( 'Header & Sidebar Background Color', 'twentyfifteen' ),
-		'section' => 'colors',
+		'label'       => esc_html__( 'Header & Sidebar Background Color', 'twentyfifteen' ),
+		'description' => esc_html__( 'May only be applied to the sidebar on wider screens.', 'twentyfifteen' ),
+		'section'     => 'colors',
 	) ) );
 }
 add_action( 'customize_register', 'twentyfifteen_customize_register', 11 );