I recently was closing my car door and accidentally hit my head. I noticed slight red tinge to the mucus but I also have a slight cold. What is it?

Blood in mucus. Unless the force of the blow to the head was hard enough to knock you down or out, the bleeding is very unlikely to be anything serious. My best guess is that you may have sustained some force to your jaw and bruised either the gums or the mucous membranes in your mouth against your teeth. A cold is very unlikely to cause bloody mucus;.
Mucus. I doubt the car door accident has anything to do with the blood tinged mucus unless you hit your nose. The "cold" probably irritated your airway mucosa enough for some slight bleeding.