PS script to change desktop background color for all local users doesn't work

hi friends

I am using the following PS script so that any new user which I create in my system (win2008R2 or windows 7 or 8 or...), his desktop background be set as solid color type & its color be red

$mypath="Registry::\HKEY_users\.DEFAULT\control panel\desktop"

new-ItemProperty

$mypath="Registry::\HKEY_users\.DEFAULT\Control Panel\Colors"

but this (line 3 & 4) don't work & any new user I create & login & logoff multiple time, all their desktop is default blue color.

but the same code works when I apply it in HKCU path.

I really need this to use in my training center with lots of students.

any help please?

thanks inadvanced