The color of pleural effusion is yellow and then red may be due to the decrease of pleural fluid, tuberculous infection, broken blood vessel wall and other reasons. 1. Decrease in pleural fluid: Decrease in pleural fluid may cause the color of the fluid to become darker and appear red, and it is necessary to go to the hospital for pleural fluid examination in time to clarify the reason for the change in the color of the fluid. 2. Tuberculosis infection: such as tuberculosis pleurisy may be due to tuberculosis mycobacteria and other germs, into the pleural cavity leading to the emergence of infection and thus deepening the color of pleural effusion, there may also be chest pain, dry cough, dyspnea and other symptoms. 3. Breaking of blood vessel wall: It is possible that the blood vessel wall breaks down due to long-term pleural effusion, causing the color of pleural effusion to change from yellow to red. It is recommended that patients go to the hospital in time to confirm the diagnosis, so as not to cause delays in the condition leading to aggravation of the condition.