I think I've got the hang of it (and I must have understood this onceuponatime),
but can someone explain why:

media{scattering{1,<0.1,0.7,0.7>*med_fact}}

tends to give red media for high values of med_fact (e.g. 0.1)
but turquoise for lower values (e.g. 0.01)?

I expected turquoise for all values.

I assume that my basic misunderstanding is that the colour defines what colours
to absorb/scatter and what colours to leave alone, so high values tend to end up
absorbing all the blue and green light, leaving only red, but I'm still a tad
confused as to why low values give turquoise.

Here is my whole test scene:

// Persistence of Vision Ray Tracer Scene Description File
// File: ?.pov
// Vers: 3.5
// Desc: Basic Scene Example
// Date: mm/dd/yy
// Auth: ?
//

#version 3.5;

#declare med_fact   = 0.01;
#declare cont_rad   = 30;

global_settings {
  assumed_gamma 1.0
}

// ----------------------------------------

camera {
  location  <0.0, 0.0, -25.0>
  look_at   <0.0, -10.0,  0.0>
}

sky_sphere{pigment{rgb 1}}

light_source {
  <0, 0, 0>
  color rgb <1, 1, 1>
  translate <300, 2000, -300>
}

// ----------------------------------------

#declare Sea =
sphere{
  0,cont_rad
  pigment{rgbt 1}
  hollow
  interior{
    media{scattering{1, <0.1,0.7,0.7>*med_fact extinction 1}}
  }
}

#declare Bed =
plane{
  y, -10
  pigment{rgb<0.9,0.9,0.6>}
}

#declare Wall =
difference{
  sphere{0,cont_rad}
  sphere{0,cont_rad - 0.01}
  plane{y,-12}
  plane{-y,-12}
  pigment{rgb 1}
}

object{Sea}
object{Bed}
object{Wall}

--
#macro A(V,B,C,R)#while(B-256)#if(V-128/B>=0)sphere{0,.5translate<C-4R-1,9>
pigment{rgb<1-C/8R/2C/8>}}#local V=V-128/B;#end#local B=B*2;#local C=C+1;#
end#end A(234,1,0,2)A(85,1,0,1)A(81,1,0,0)light_source{-5 1}//Tom Melly

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Trying your example out here and I can't explain it. I looked at the scene
differently by moving the camera back outside of the dome. It really makes
for a better perspective I think. To do that, the "wall" must be commented
out. I can get both red and cyan showing in what appears a odd sort of way,
somethng about the light interaction/shadow. I'd never have expected both
colors to be present in a single color media, very unusual to me. Maybe
someone know of the reason for this but a color shift like this is
practically the stuff of wavelengths. To guess, might it be a
self-shadowing?

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I didn't experiment with the scene, but as I see you
haven't set any methods, intervals or samples, it might
have to do with them. Perhaps self-shadowing and
strange interval placement cause this, more intervals
(when using method 2) or more samples (using
method 3) might do it...

--
Tim Nikias
Homepage: http://www.digitaltwilight.de/no_lights/index.html
Email: Tim### [at] gmxde

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