If the bar at the top of your Live View changes color it's measuring the quality of your device's connection to your Wi-Fi Network. The three colors the bar can turn are:
Green: This indicates that you have a strong connection to the network and should experience no problems using the Live View function.
Yellow: Your network connection is weaker than it should be. You may experience some problems with your Live View.
Red: Your network connection is extremely weak or sporadic. You may experience severe problems including a black screen with your Live View.