As Brett Champion says in his answer the NaturalColor stylesheet is still availble but hidden. If you want to use it on a regular basis, I suggest moving a copy of it to a folder where Mathematica will find and put it on the Format > Stylesheet menu.
Mathematica can aid you in finding the stylesheet folders and the stylesheets themselves. The function
findStyleSheetFolders[] :=
Module[{userStylesheetFolder, wolframStylesheetFolder},
userStylesheetFolder =
FileNameJoin[{$UserBaseDirectory, "SystemFiles", "FrontEnd", "StyleSheets"}];
wolframStylesheetFolder =
FileNameJoin[{$InstallationDirectory, "SystemFiles", "FrontEnd", "StyleSheets"}];
{userStylesheetFolder, wolframStylesheetFolder}]
will find the required folders. wolframStylesheetFolder
is where WRI installs the stylesheets WRI distributes with Mathematica. userStylesheetFolder
is a good place to install a copy of the NaturalColor.nb
stylesheet you use so it will appear on the stylesheet menu.
The function
findStyleSheets[] := FileNames["*.nb", findStyleSheetFolders[], ∞]
will show you all the stylesheets installed on your system.
On my system, evaluating
findStyleSheets[] // TableForm
gives
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/ArticleClassic.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Article/JournalArticle.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Article/LaTeX-Article.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Article/LaTeX-Report.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/ArticleModern.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Article/Preprint.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Book/Compilation.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Book/LaTeX-Book.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Book/Monograph.nb
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Book/Textbook.nb
...
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Creative/NaturalColor.nb
...
/Applications/Mathematica.app/SystemFiles/FrontEnd/StyleSheets/Wolfram/Reference-zh.nb
/Users/oldmg/Library/Mathematica/SystemFiles/FrontEnd/StyleSheets/Notebook.nb
/Users/oldmg/Library/Mathematica/SystemFiles/FrontEnd/StyleSheets/OReilly.nb
As you see, I've installed two custom stylesheet notebooks in the user stylesheets folder. These custom stylesheets show up in my Format > Stylesheet menu. If you move a copy of NaturalColor.nb
into your user stylesheets folder, it will be visible in your Format > Stylesheet menu.