Digital BW, The Print

> It may not be the "same thing" but you certainly can get the EXACT same
> results in your final grayscale output.  You can do manipulation to the
> grayscale data and arrive at the EXACT same resultant output as you can by
> manipulating the RGB data then converting to grayscale.

Let say there's a grayscale image of an 18% blue patch, an 18% red patch,
and a greyscale strip.

With the channel mixer and an RGB image, I can alter the tonal values of the
blue and red patches, making one darker and lighter without affecting any
changes on the tonal values of the greyscale strip.

In a greyscale image, the colour patches appear identical, both middle grey.
I can manipulate the greyscale data, but it makes no distinction between the
red and blue patches because they are both the same grey value. If I change
the value of one colour patch, the value of the other colour patch is also
changed in the same manner, as well as the corresponding grey patch on the
greyscale.

How can you manipulate the greyscale image to separate or blend different
colour values as you would via the channel mixer, if there are (obviously)
no colour values in the greyscale image?