When I saw the thread title, I initially assumed that you meant Colin was Jeremy's accomplice and clicked eagerly to see how you had come to such an unusual conclusion. It is slightly deflating to find that this is not the case, but I did not know that Colin held the view that there was an accomplice.
Among the people we know of, I suppose the obvious suspects for an accomplice would be Brett, Julie and Matthew. In the case of Julie, an accomplice need not be present, and in the case of Brett and Matthew, their alibis are less-than-perfect.
Julie is still my favourite for an accomplice. It occurs to me that her statement about Matthew McDonald may have been a ploy to further distance herself from the crime when she turned on Jeremy. She is suggesting that, indeed, Jeremy had an accomplice: someone else.
But I assume that when Colin refers to an accomplice, he means someone actually on scene, assisting Jeremy in the shooting. Brett did morally corrupt Jeremy. What was the feasibility of him sneaking back to the UK? Would he go to those lengths? Did Matthew's alibi really stand up or was it more a case of the police being dismissive of Julie at that precise point?