Our team has used green LED's in the past. I believe the reasoning is that green lights have a slightly higher intensity therefore any reflections are easier for the Axis camera to find. We are trying to use a green LED ring because of the issues we have had with white ones. Our previous white ones have had an issue being recognized by the Axis camera after being reflected. A lot has to do with the amount of outside light that may interfere with the camera's readings. Personally I would suggest getting a green LED ring just because of the issues I have seen with white ones.
As for the diameter, personally I suggest getting the 60mm because it fits around the axis camera perfectly. We are still trying to find a good way to mount it. Our team may end up just using double sided tape if we cant figure anything better out.
Hope that helps
Kevin