Why does one of my Google Hangouts contact icon color change for no reason from green to blue?

[AC Question]

One of my contacts in Google Hangouts -- their icon will be green which is the "normal" color. Without making any changes what so ever on their end - it will go to blue. I have asked to be certain they are not being hacked still not sure and I am just curious if anyone might know the answer. Seems very odd to me. No changes on wifi or using data. We have even tried different devices and that isn't the answer. But there must be one. Thanks in advance!