Depends on cause. It really depends on the cause of the effusion. For example, it can look very bloody if related to a hemothorax or complicated parapneumonic effusion, it can look like pus if due to an empyema, or it can even look milky if related to a chylothorax, in general, if due to pneumonia or malignancy it usually looks clear to yellowish with some blood.
Varies. Most are yellow, pale serous. Hemothorax is bloody -red chylothorax is creamy pus can be yellow, green, nasty.
Depends. Caused bybwhat? Trauma- bloody Cancer- bloody or yellow serous, or sero- sanguinous. Infection - bloody or cloudy or pus or serous.