The varying colors of the contacts is not random on my phone (Galaxy S5).
Each time I add a contact it selects the next color in the sequence...
Blue, Green, Orange, Orange
Blue, Green, Orange, Orange...
It just looks untidy in the Contacts list, and I see no way of changing it - apart from adding a contact a few times until it's the color you want, and deleting the unwanted ones. You could also create some simple color pictures to assign to each profile picture (if you don't have their photo)