The change from white phlegm to yellow phlegm is not usually easy to cough up, but may be a sign of disease progression. Yellow phlegm is not easy to cough up, but is more viscous and clings to the bronchial tubes or throat, making it more difficult to cough up. White phlegm is generally less viscous than yellow phlegm, and if white phlegm is not easy to cough up, the progression to yellow phlegm may worsen the manifestations of the disease. In addition, if the sputum starts out white and gradually becomes yellow, this may also be a sign of disease progression. This is mainly due to lung infections and chronic lung diseases. As the infection worsens, the sputum may turn yellow, and the chronic lung diseases may cause the sputum to be unable to be discharged in time, and it may gradually accumulate and turn into yellow sputum. It is recommended to seek medical advice and treatment under the guidance of a doctor.