You are but for the examiner who is correcting it you should use black and blue , it's just easier to read :)
the examiners use red, green and purple (i'm sure i read that somewhere) so try to refrain from them. use darker colours, as they are easier to read. if you make a mistake, put a line through it and bracket it, rather than tippex, as my history teacher said you can get marks if it was correct
My Irish teacher was complaining about how it's really straining to read pink or green or purple pen when you're correcting 300+ papers in blue/black. If it makes things easier for you then yes definitely use whatever colour you like but if the examiner isn't happy correcting your exam becaus of the pen colour, they might not be as generous with marks. Just a thought.