I did some research on this and found that it seems to be random. According to kdmrc, the default icon is found in /opt/kde3/share/apps/kdm/faces/.default.face.icon. However, there is another file called .randomlist with the following content:
Code:
User-blue
User-purple
User-red
User-green
I guess unless you specify a certain user icon, you will get this random distribution of colors.