Been fooling around with glows and ran across an interesting
property... negative colors can be used to cancel or shape the
halo.

I have included a trivial example where a negative green glow
cancels a positive one.  This action is actually color-specific so
you can selectively allow different frequencies to pass or not
as required.

Not very exciting news perhaps, but it might be useful...

-------------------------------- snip --------------------------------
sphere { < 0, 0, -2000 > 1 pigment { rgb < 1, 1, 1 > } }

camera { location < 0, 0, -10 > look_at < 0, 0, 0 > }

/* positive */
glow
{
  0.5
  glow_type 0
  color vnormalize( < 0, 1, 0 > )
  location < -1, 0, 0 >
}

/* negative */
glow
{
  0.5
  glow_type 0
  color vnormalize( < 0, -1, 0 > )
  location < 1, 0, 0 >
}
-------------------------------- snip --------------------------------

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