2 weeks ago i slammed my finger in a door and now there is grennish/yellow pus coming from it, why? there is a little swelling but little pain

You : You have likely defeloped an infection. You need to seek medical care right away. If an infection is the cause then you will need antibiotics and diagnostic testing to find out what type of bacteria you are dealing with and what medications will work. (this is called a wound culture and sensitivity). Take care.

Seek : Seek medical care you may have an infection. You may also just have a wound that is non infected and draining serous fluid. Nonetheless that is not a decision for you to make, go see someone. In the mean time there is no harm in soaking your finger in some warm (not hot) water and hydrogen peroxide, applying some antibiotic ointment and even splinting your finger ( without being too tight on the circulation) splinting or limiting motion helps stop the spread of infection and may make you feel comfortable too. Infected fingers can hurt especially when the joint or tendon moves. Finally you could have a blister form swelling from a fracture, the last thing you want to do is get that infected even if it is not now. Get an x-ray too don't assume there is no bone injury.