Who I have the same questioned this topic

As you can see, in the group below, one person has a blue chat bubble and the others are grey.  The one with the blue is the only one who does not receive the messages to his phone.  Please do not ask me a bunch of questions about versions of apps or who our carrier is.

i have just one simple question: what do the different colors signify exactly?

Also, please do not guess at it.  Somebody created this app and told it to display the different colors and that person should have told other employees what they mean.  It should be written down somewhere.  Hopefully Verizon has competent employees who are capable of answering this one simple question.