UI Mesh stays black

I'm trying to draw some of my UI elements with meshes, but they are always drawn black. material.SetColor() does nothing and setting the color of the canvas renderer also doesn't work. Moreover, the Editor only shows the mesh if look at the UI canvas from the back side. The material I am using is white colored and its shader is the standard one. I also tried some other shades, which either display nothing at all or the same thing. Is this a bug or am I missing something?
UI Mesh stays black