Unexcepted element color after exporting view as image

Hi, I set my material color to be gray as below.

What I am doing is to export elements into images (i.e., each element has an image). I used Dynamo to create views for each element and then exported the views into images. But in the exported images, I found the slab colors are not gray, as shown below, which are different from the gray color I set (same to the wall). How to make the color of the exported element image the same as what I set (gray) in the model? Thank you!

Dynamo scripts I used:
export image by element.dyn (44.0 KB)

Revit model: model.rvt - Google Drive