Yellow phlegm and viral infection and upper respitory infection...?

i was diagnosed with this and has been 10 days now i feel alot better no fever no headache only symptom i have left is coughing up yellowish phlegm in the mornings and a lil dry cough but gets better after the morning i was just wondering why is it yellow now and if its from the infection coming out of me... i heard viral infection can last up to 2 weeks or more...