In the Custom Proof setup box, there is an option Simulate Paper Color to display the simulation on screen.
However, as Michael Reichman quotes:
Soft proofing is simply a mechanism that allows you to view on your computer monitor what your print will look like when it is on paper. A specific paper. That paper and ink combination has been defined by the profile that you or someone else has made for your printer / paper and ink combination. When a printer profile is made the colour of the paper is one of the factors that is figured into the profile, because the spectrophotometer is reading the combination of the ink, and the paper that lies beneath it.
Bolding added.
So, if the paper color, brightness, etc. has already been incorporated into the profile because the profile was created using that paper, what does the Simulate Paper Color option do? /Dan