Ableclean, when you say grey do you actually mean grey? Or just a different shade colour of the white? It could just be you brought the dust to the surface and it greighs the sight. As formula mentioned, it would be advisable to finish your wand stroke all in the same direction, You should avoid the chevron effect as much as you can. Some velvet carpets and especially silk and linen carpets will not recover from the chevrons if left to dry that way. Other thing is, you mught want to brush the carpet after wanding it which will mostly always get out the chevrons. Best, Dave.