Removing reflection and diffusion from polygons with an image

I have a few polygon entities which each have an ImageMaterialProperty as their material.
The issue is that as you move/pan around, the polygon image ‘changes color’ due to the reflection/shininess of the material.

How does one make it so that the material doesn’t reflect light, i.e., isn’t affected by the viewing angle a user chooses to look at it?

Update: by giving an image some transparency (even just 1%) seems to remove any color distortion.