I want to take one of the brass textures and layer a little oil and grime on it.
I thought this would be simple but I'm goofing up something very basic. I
thought I'd start with a completely transparent pigment, just to get the hang of
the syntax. Layering a transparent pigment should do absolutely nothing, but
instead my brass turns grey, roughly the background color.

#declare Plate  = box {<-4, .13, -4> <4, -.1, 4>
   texture {
     T_Brass_2A
     // finish { specular 0.90 roughness 0.21 ambient 0.05 }
     pigment {
        color rgbt <0,0,0,1>
     }
    }
 }
 Plate

....plus a skysphere of grey and an include for metals.inc. What am I not
understanding? Note that if I use an rgbt of 1,1,1,1 I get almost white (why?),
and 0,0,0,0 gives almost black (which makes more sense.)

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