In most computer-based exams a pen is not needed at all — everything is on screen.
For offline OMR exams, centres usually provide the required pen or specify a blue or black ball-point pen on the admit card.
Carrying your own pen is generally allowed if it matches the instructions, but extras like pouches are not.
Read the admit card instructions — some exams strictly allow only the pen supplied at the centre.
When in doubt, carry one simple transparent pen and nothing else.