Personally, I would change the cloud media to the following (in 
particular making them white; colour differences should come from the 
light and/or environment) and increase samples a bit to reduce artefacts:

box {
   <-1,-0.5,-1>,<1,1,1>
   pigment {rgbt 1}
   hollow
   interior {
     media {
       samples 10
       emission <0.9, 0.9, 1>*0.1
       absorption <1.0, 1.0, 1.0>*2
       scattering {1, <1, 1, 1>*1 extinction 1.0 }
       density {
         gradient <1, 0.25, 0.5>  // controls the cloud formation
         turbulence 0.5
         frequency 5
         color_map {
           [0.0 rgb 0.0]//border
           [0.5 rgb 0.1]
           [1.0 rgb 1.0]//center
         }
       }
     }
   }
   translate <0, 1, 0>
   scale <5, 1.5, 5>
}


-- 
Thomas

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