When you start composing your tweet, the text will change colors when you’ve added something clickable, like a username or URL. The text changes to blue, and this is to let you know that you’ve done something “different.” It’s nice onboarding for new users, or when you’re quickly typing out your latest amazing thought.
Similarly, the text will change color if you’ve gone over 140 characters, in red, showing you exactly which ones have to go.
