Selection effect on WPF 3D model

Here's one possibility that comes to mind:

In each GeometryModel3D, set the Material property to a MaterialGroup.  Put whatever materials you're normally using into that group, and then add a final material:  An EmissiveMaterial with a Brush property set to Black.

When the item is selected, you can paw your way through into the GeometryModel3D, find the last object in the MaterialGroup, and set the Brush property to Brushes.Gray. The model will then brighten in color.  To make it bluer, you could set the brush color to the RGB value 00-00-40 or something like that.