I wanted the Brood to look very different. so I gave them light grey skin and black bone plates and purple markings on hands and feet.
The Brood was originally created by humans as bio-manufactured soldiers. And although this is long forgotten by most on ATTR this would still be part of their culture.
That is why I gave them a number on their heads. This would have been a pratical way for their human masters to keep track of their troops in battle and it have carried over through the centuries, its original intent lost but kept only as a tradition.