Code to change the color of the Dialogflow widget:

<style> 
df-messenger { 
--df-messenger-bot-message: #ff09ff; 
--df-messenger-button-titlebar-color: #ff09ff; 
--df-messenger-chat-background-color: #fafafa; 
--df-messenger-font-color: #fff; 
--df-messenger-user-message: #1187ee; } 
</style>

Code to add to your functions.php:

// Add scripts to wp_head()
function child_theme_header_script() { 
  	if (is_page('Contact')){ ?>
	     // Paste Dialogflow Messenger code here
	<? } else { ?>
	     // Paste Dialogflow Messenger code here	
	<? }
}
add_action( 'wp_head' , 'child_theme_header_script' );