plz help!!! i have a sputum test monday.. i have ben coughing up quite alot of...

[Greg S]

...yellow brownish sputum.. i have? the bottle home with me.. and i have to cough up the stuff and bring it in monday.. but what i don't get it is the first time i coughed up it was yellow and then it dried up so i emptied it out.. and now when i need to do it again i can't cough up anything.. just white sputum.. how come i can cough up yellow and brown sometimes and others i can't.. why is this.. is it because im not hocking up hard enough to get it up