This is a great question. I have studied the differences and can visibly tell exactly what you are saying. Nevertheless, there are colors which look the same.
For a good grey use: 12369084
There are other colors that I was able to find.
However, the best thing to try is this: If you want the Cluster to be the same color as the tab control, make the background of the cluster transparent. When you are changing the color hit the space bar to select between border and background and hit the the "T" in the top left corner of the color change box.
I hope this helps,
Rick Kuhlman
Applications Engineering