For the past 17 years, Google has constantly change their logo from time to time. This  announcement was made on Google Blog and below is a quote from the blog

Today we’re introducing a new logo and identity family
that reflects this reality and shows you when the Google
magic is working for you, even on the tiniest screens. As
you’ll see, we’ve taken the Google logo and branding,
which were originally built for a single desktop browser
page, and updated them for a world of seamless
computing across an endless number of devices and
different kinds of inputs (such as tap, type and talk).

It doesn’t simply tell you that you’re using Google, but
also shows you how Google is working for you. For
example, new elements like a colorful Google mic help
you identify and interact with Google whether you’re
talking, tapping or typing.

Meanwhile, we’re bidding adieu
to the little blue “g” icon and replacing it with a four-color
“G” that matches the logo.

Google has been known for changing Logo and this definitely won’t be the last. Watch out has the new design will roll out across all Google platform soon.

Source: Google