When cold or flu germs infect the nose, the nose makes mucus to help wash the germs away. At first, the mucus is clear. But as the body's immune cells fight back, the mucus begins to change color. First it becomes white or yellow. Later it may become green. This is normal and does not mean you need an antibiotic. Still, if things don't improve after about 10 days -- or if your symptoms are severe -- see a doctor. It's possible that you've developed a sinus infection and need antibiotics.