My laptop has a yellow USB port. Is it USB 3.0?
If it's a Lenovo ThinkPad chances are it's the "always on USB", which is a special port that remains powered when the computer is asleep, or when the computer is fully powered off and the AC adapter plugged. This allows you to, for example, keep charging your phone or wireless gamepad without having to keep your computer running.
It can be whatever colour that your manufacturer wants it to be - I do not think there is a standard.
That being said, every product I have seen that supports USB 3 seems to have a blue socket, so there may be one!
If it is yellow, my guess is that it may be a way to indicate enhanced/increased/special power output.
What model do you have? If you write the model number, I can take a look to be sure.