As scifan said, the colour of the dock holding the bottom row of icons will change with the colour of the wallpaper.
If you look at this screen:
Where the background is black (bottom left hand side) you will get that natural grey colour. But as the screen colour changes, the grey starts to take on the colour of the screen (in the middle). The lighter the colour of the screen (bottom right hand side), the more that grey of the dock will change to the colour of that part of the screen.
So, a very dark colour will give the "normal" grey. A very light, bright colour will give much closer to the screen colour on the bottom icon dock.
And I am going to stop now as I think I am on the verge of confusing myself as well as everyone else
Anyway, hope that helps a bit.
Andrew
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