For a second I thought this was about kids. With my sons cleft issue he gets all colors. Tonight's was blue as he ate some blue Italian ice. He has had all colors in the rainbow or a box of crayons.
I'm also bad with my allergies. Mine will turn green during heavy pollen times. Enzymes contain iron which gives off a greenish color. The longer the mucus is in your sinuses the more likely it is to be green. That is why your morning blowing will be green.
Now granted I am not a doctor, if you feel a visit is needed by all means go. What I know is through my own research and asking a doctor or two.
A side note, what a few doctors have told me is that an antibiotic for minor infections will do nothing against the virus. They told me Sudafed would work much better. But again, take it how you will and if you feel the need to visit do so.