By @smartavionics
I don't understand why it only seems to work if I define u_color via [default] andd why it doesn't work when I set it inside the shader itself.
Somewhere between the xray.shader and the xray_composite.shader the colors are stored as unsigned int. Since we want to count the integer number of faces behind a pixel in the red channel, we simply use a color with an r value of 1/255.