Just click on the PaletteTool icon.  PaletteTool will detect the number
of pens available on the default Public Screen, and open it's window on
this screen.  Note that the Workbench screen is the standard default
Public Screen.

My PST (Public Screen Tool) program can be used to open a Public Screen
at any AmigaDOS 2.04(+) supported resolution and depth.  This includes
any graphics boards which add entries to the Amiga's display database
(like the EGS Spectrum). 

PST is available on many fine BBS's.

I strongly recommend you get (and use!) PST if you are interested in
running programs (like my Palette/LyapunovSpace/Terrain programs) on
public screens other than the Workbench.  Like PaletteTool, PST is in
no way dependant/tied-into my other programs.  It is a full, stand-alone
utility in it's own right.