Digital BW, The Print

Ernst,

I've noticed what you describe (I think), so I checked Photoshop6 
Channel Mixer and here's how my version works:

With an 8 bit RGB image, select Channel Mixer, click Monochrome, then 
un-click it. The image loses color but the file remains an RGB file with 
the same data in all three channels.  When Monochrome is off, you have a 
choice of R, G or B channels in the Output Channel menu instead of Gray 
when Monochrome was checked ON, but there doesn't really seem to be any 
difference among the three channels when switching the Output Channel 
colors.

I suppose that, if you were to need a monochrome version of a color 
image to layer into another RGB file, this is one way to do it. I don't 
see any other use for it.

Regards,

Frank