Many people believe that the color of your mucus will tell you what kind of infection you have. This just isn't true.
When you're sick, it's possible to have clear, yellow, or green mucus. In fact, you usually produce clear mucus at the beginning of your illness. As your body starts to fight off whatever's bothering it, your mucus changes color. Yellow and green hues may be caused by certain bacteria, or come from the enzymes your white blood cells release when they're fighting an infection.
Your mucus may even change color throughout the day. It can be one color in the morning but clearer in the afternoon. (This is because mucus accumulates and dries while we sleep, but once you start moving around, your mucus can flow normally again.)
So while the color can't tell you exactly why you're sick, it's still a sign that something might be up.