Dr. Lekutat's answer:

In infections of your upper airways your body tries to fight off germs, like viruses or bacteria. And it does it partly with increasing the blood flow in the infected areas, namely the mucous membranes in your nose or throat. A clever idea, as an increased blood flow brings in all the white blood cells, responsible for a big part of your immune response. The mucous coming out of your nose therefore comes from the mucous membranes and is a sign of an effective immune system. The colour change of the mucous is due to the amount of white blood cells in it – more with bacterial infections than in viral diseases.