What causes yellow mucus in your throat and mouth?

Throat mucus: Mucus tends to thicken and turn yellow or slightly green as water evaporates from it. The flow of mucus slows at night, so that in the morning you'll feel the mucus in the throat, and there is an urge to cough or expectorate it. You can relieve this, by staying hydrated and using a saline rinse to help thin mucus and improve mucus clearance.

Mucus membranes: Usually secrete a clear mucus to lubricate the membrane. Yellow mucus may be an indication of an infection.