The updates are mostly cosmetic and designed to unify user experience between the Tweet button and the later released Follow button. Both buttons are now light grey with the original aqua blue Twitter Bird icon replaced by Twitter’s more official and truer blue colour branding. The change in blue colour is the most obvious change made to the buttons’ overall design. Text colour has changed from blue to black.
With the Follow Button, the account username has been brought inside the button itself (previously placed outside) and the follower count inside a bubble to the right – exactly like the Tweet button.
Twitter has also made small changes to both buttons under the hood, citing “improved mobile and retina display support for the iPhone”