What does dark mucus mean?

I have dark brown sometime green colored mucus almost every morning. This has been going on for a while and I've though nothing of it, but recently after I eat a meal or snack I cough alot. I feel like something should be coming up, like it's just in the back of my throat, but nothing. I'm not a smoke. 2 years ago I had a strep throat and series of other illnesses at once, but I think it's unrelated. Also, when I was younger I had bronchitis off and on for a few years. Although, that was more then 10 years ago. Any ideas?