First off, you'll need to have the ScrollColors plugin installed.
Then you need to fill up your
~/.vim/colors directory with as many color schemes as you can find.
Are you ready?
Open a new Vim window and enter the following command, which defines an autocommand for the WinEnter event that applies to all and triggers the ScrollColor command for cycling to the next color:
:au WinEnter * :silent CN
Then, set the 'mousefocus' option, if it is not set already.
Now all that is left to do is make some splits:
Ctrl-W S or
Ctrl-W V (a few times).
And then you wave your mouse around and have a good time.