How do I change the dock color on iPhone?

Go to Settings->General->Accessibility->Reduce Transparency, and make sure its turned off.

This will make the grey backgrounds on the dock and the folders a bit translucent and as such take on color from the background being used.

But there is no way to actually modify the color of the backgrounds directly. There has never been a way to do this.

You can't change the coloring of the dock or the background color for the folders.

I have the same issue and also the notification banners as well. iOS 12 off to a great start!!

Were we able to change it in previous iOS versions? Mine was white for as long as I can remember.

The colour of the dock is largely determined by the colour of the wallpaper behind it.