Twitter developers have revealed that their Tweet and Follow buttons would get an update this week.

They are deprecating the aqua blue color to stick with grey colored buttons.

There are design changes to both the buttons. Follow button would now feature the Twitter account inside the button itself.

Here is a sample:

Users of these buttons would not have to change anything. Their existing buttons would be updated automatically.

Enhancements:

Improved mobile and retina display support for the iPhone.
Switching from button sprites to HTML & CSS which will bring better accessibility, readability, and language support for more locales.
A visual refresh that unifies the Tweet and Follow Buttons into a similar look and feel. The new button will be grey with a blue Twitter logo. The aqua blue Tweet button color will be deprecated.
The Follow Button will bring the @username inside the button, and place the follower count inside a bubble next to the button, just like the Tweet Button. This only applies to buttons that are already showing the @username or follower count.