+1  A: 

Could it have something to do with pixel snapping?

Cameron MacFarland
Unfortunately that is not it, but it sounded likely. Good background though, I was not aware of pixel snapping.
Ryan ONeill
A: 

Are they in a grid? It could be related to http://stackoverflow.com/questions/267364/wpf-eliminate-transparency-between-grid-cells#267954 which we never really resolved.

Donnelle
No grid in use, just a canvas. But they seem linked somehow. I'll have to make it simpler I think with blocks of colour and see if I can get a sample up to reproduce it. Could be linked to DPI somehow though as only happens on certain systems (not mine obviously).
Ryan ONeill
A: 

Had similar problem when used opacitymask

David
A: 

You could ask one of your tech-savvy users to run Snoop while the problem is showing, then:

  1. Click on the binoculars
  2. Put the mouse over the artifact (vertical bar) and press Shift-Ctrl
  3. Look in the Snoop window and tell you which item is hilighted

This will tell you exactly what control is producing the color you are seeing in the artifact, which may be a big clue in fixing it.

Ray Burns