Thanks for the information.  Maybe I'm slow but I don't understand what you are getting at when you ask what the colour is?

 Is this the sentence you are referring to?  " Where excavated, the "yellow ground" gives way at depth to fresher "blue ground", and deeper still, to the unaltered dark greenish-brown kimberlite.

 If so, we already know that they have diamondiferous kimberlite and I am missing your point.

 Can you explain for this humble fool?

 BGT