Why am i coughing up green/brown mucus?

OK so I am a smoker, however most of my friends talk about coughing up black chunks and that hasn't happened to me. My best girl friend told me that my smoker cough sounded like hers, and she just found out that she has asthma. neither her or I have ever had any signs of it when we were younger. so my question's is the greenish brownish mucus coming from asthma or from smoking?