This is not simply a recolor of the portal gun, additions and details have been made to improve "realism". For example, certain edges have not been fully recolored to improve the "paintiness" of it.
Click and enlarge the screenshots to see what I mean.
Copy and paste the folder, "materials"(without the quotation marks) in:
If a window appears asking to merge or replace files and/or folders, click the "yes to all" selection.
Afterwards, start up Portal and your portal gun will be red!