But that one red channel is adjustable, so its infinitely variable. What's more, one can adjust the tonality of the color original in other ways -- shadow/highlight, curves, selective color, color replacement etc. -- so almost any grayscale variation is possible.
Indeedy; there are two facts to bear in mind here:
1. It's almost always possible to get exactly the grayscale image you want by tweaking things in Photoshop, and
2. It's almost impossible to *exactly* duplicate the effect of a given physical filter in Photoshop. It's *absolutely* impossible to simulate things like UV, polarising and deep red filters, but you can usually get the effect you wanted (eg a darker sky or improved contrast) a different way.
S
