D.P.
Although I have read recently that green is NOT a sign of infection, as once commonly thought--it seems to me that if the green is accompanied by some fever--it's a sinus infection.
My son seems to get the 10 day "cold" followed by sinus infection shortly thereafter. It's his pattern.
A cold should not last more than 10-14 days, so if it's longer than that & green, I'd bet sinus infection.