Please refer to section 5.1.7 'Stationery, materials and other equipment' in the Cambridge Handbook. It states:
“Candidates must write their answers legibly in black ink. Candidates must not use red or green ink.”
Candidates may use pencils for mathematical constructions and graphs. Otherwise blue or black ink should be used as (a) it can’t be altered after the examination and (b) the examiner can see the way the student was developing the question even though the final answer may be incorrect.