A tube is different colour, why?

I saw a similar unit installed on a roof and one of the tubes was a different colour, like the black material in the tube had broken up, what is the black material, does this happen often?

The black material is a black heat (Infra Red) absorbent compound with a silver inside.

What you are describing is a faulty tube, this happens when the vacuum in the tube is broken, the tube will most commonly turn a milky white.
At this stage the tube no longer absorb heat efficiently.

We haven’t experience the black material cracking up in our tubes, but our tubes are specifically manufactured for us.

Our tubes very rarely experience failure as they are inspected before they leave our warehouse (yes, every box is inspected by a real person in our warehouse in Wallan Australia).