Incoming Bubbles in iMessages are different colors.

Has anyone figured out why some of they're contacts' bubbles are different colors?

Some of my contacts' are still the original gray, while others are pink, dark gray, purple and what not.

I checked the grouping of these contacts in Contacts app, but there's no rhyme or reason for them to be different. I thought that they were color by group. Like pink being "family", dark gray being "friends", purple being "co-workers". But this is not the case.

Even what is being used for iMessages, email/phone number, is not a factor.

KOT