???

I had no idea there were different colour variants of these trucks either. The dark grey ones look more similar in colour to normal Plarail train wheels or the bodies of the Troublesome Trucks. I actually got two HiT Toys ones in this coal loader set (they were not pearl grey as depicted on the box):



I used one of them in my Ivor the Engine remake since I didn't have the coal blocks for regular trucks (Ben was sold here back then, but Bill wasn't). Then again, I once used black LEGO parts and they looked better. BTW, the lack of consistency between the shades should not bother you at all. As Nigels wrote, it adds a bit more realism when they look different from each other.