Barsabbas and Silas are specifically described as being prophets in Acts 15:32 and that is our main description of these two men. Granted, there isn't as much on Barnabas, but it makes sense to print them as Green for prophets. Peter...is always an interesting case. There are quite a few characters (David, Moses, Peter, even some lesser-knowns like Deborah) who easily fall into different 'themes' and could have many brigades printed for them. Peter could be Purple/Clay/Green without any question, sure. But unlike people like Barsabbas and Silas, that is not how he is
known. His main jobs in the bible, and how he is described to us, are being a Disciple and being a leader of The Way after the crucifixion.
We don't need to print every prophet in Green, but for those who are described mainly as being prophets (like Barsabbas and Silas), it makes sense.
It's not an omission on Peter, by any means, but just part of the fact that not everyone gets every brigade that could be attached to them. We don't need a Red/Green/White/Purple David (though that'd be cool IMO...), we have King David to represent the Purple, for instance, and a Red or Green version to represent the other aspects printed at different times. And we don't need a Gold/Green/White Moses (especially not that ability...), he's known mainly for being a judge, just like Deborah doesn't need Gold/White/Red/Green because she is known as a judge, so Gold.
While a character may 'qualify' for a brigade, there should probably be a reason to
include it rather than exclude (meaning a better case for giving them multiple brigades than have to come up with reasons why they wouldn't fit). Which means, that when we have the following being true...
Because he was awesome enough already?
...then we don't need to add Green for some HT and prophet abuse