OSX color picker history - save file data
In my investigation it seems the preferences are saved here:

~/Library/Colors/NSColorPanelSwatches.plist


It is my guess that this file would be overwritten on a clean install.  However, if you want keep it, from here on out you may be able to copy it to a safe location.  If you have a Time Machine backup, you may be able to dig through it and recover your previous version of this file.

If you named your custom palette, then in that folder would be a file similar to mycustompalette.clr