The logo was not changed until 2010 when the slightly orange circle in Google was made a definite orange that was clearly visibly as orange and not a yellow color. 2013 and 2014 saw very minor changes but in 2015 Google switched to a modern logo on a preface called Product Sans.
Recently Google sometimes has a theme logo if there is a holiday or they want people to recognize something around the world. It is usually national holidays or about someone that Google wants to recognize on that day.
The History of Google 