that helped i found the code. its in a file called GuiIngame.java. i dont know the file path for it but it contains a bunch of code for overlay gui effects and stuff so it should help a lot.
steps to find it in intellij are right click on a folder in the project, click 'find in files' button, search for "pumpkinblur.png" and click the scope button.
i tried doing this but cannot find anything in the source code. i looked in 'BlockPumpkin.class' and 'ItemArmor.class' but neither of them have anything about putting a pumpkin on your head. any help is appreciated.