I could reproduce the behavior in AutoCAD 2013.
While the color was darker when the Visual Style was "2d Wireframe", the color seemed proper yellow with Visual Style set to any other style, for ex : Realistic.
I am not too sure about the reason for this difference but I have come across such differences in the way the graphics is rendered with visual styles. This could be due to the different graphics engine used by AutoCAD for 2d and 3d graphics.
Here is a workaround that you can try and which worked for me :
Instead of "wd.Geometry.Draw(BlkRec)", you can try replacing it with "BlkRec.Drawable.WorldDraw(wd)"
Hope this helps.