Change saturation of a particular colour (with -modulate ?)

[magickuser]

I'm working on some colour JPEGs, and I'd like to be able to desaturate the red colour. I know that I can change the overall saturation with the -modulate option in 'convert,' but how can I desaturate only the red?

[anthony]

You could -seperate the channels and then module just the red image, before -combine them back together