So, realising that I'm not alone in not knowing how to do this I summoned up the courage to look through *all* the functions, no matter how oddly titled, and I found Opaque which does the business (arguably I shd have done this before - but I needed to have my back to the wall...).

For the record, this does it: 
use Image::Magick;

my $image = Image::Magick->new;
my $file = $image->Read('old.gif'); 
$image->Opaque(color => '#000000', fill => '#0000ff'); 
$image->Write('new.gif');

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§ George Sherston

In reply to Re: Changing image colours - Image::Magick? - my solution by George_Sherston
in thread Changing image colours - Image::Magick? by George_Sherston

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