Creating a function to change custom-set-faces

[Jon]

I have a set of custom colors set up in emacs that I find much easier
on the eyes - lots of greens & pastels.
Unfortunately, it doesn't print too nicely.  My current solution is
two .emacs files - when I want to print, quit emacs, swap the files,
start emacs, print, etc etc.

Obviously, this doesn't seem like the best way of doing things.
The .emacs file contains a section custom-set-faces, with all of my
customizations in.  I was hoping to create a function to set these for
me, so I could swap colors on the fly.

I tried M-x custom-set-faces, which doesn't exist.  And that's about
where my emacs knowledge runs out...
How can I set custom faces up, based on info from the .emacs file?

Thanks,
Jon