I would like to save only one of the color channels, for example the magenta channel of my .tiff image. How do I do that? Every time I save my image I just get a grayscale image, even though I'm able to see the magenta channel just fine.
That sounds normal to me.
The magenta channel, on its own, is effectively a greyscale image -- it only becomes magenta when you print it in magenta ink. But in the channel itself, it is simply a series of pixels from 0 to 255.
What do you intend to do with the image, when you have it?
I want to make a collage of the different color separations of the image.