documentclass[a4paper]{article}

usepackage[margin=1.0cm,noheadfoot]{geometry}
usepackage{atbegshi,tikz,lipsum}
pagestyle{empty}

AtBeginShipout{ifnumvalue{page}>0%
begin{tikzpicture}[remember picture,overlay]%
fill[gray] (current page.north west) rectangle %
++(5.0cm,-paperheight);%
end{tikzpicture}%
fi}

begin{document}

% Dummy code for first template page with vertical bar
begin{tikzpicture}[remember picture,overlay]%
fill[gray] (current page.north west) rectangle %
++(5.0cm,-paperheight);%
end{tikzpicture}%
vspace*{fill}
begin{center}
LARGE{Template page with vertical bar}
end{center}
vspace*{fill}
%clearpage

% Additional pages with sidebar and dummy text
newgeometry{left=6.0cm,right=1.0cm,top=1.0cm,bottom=1.0cm,noheadfoot}
begin{center}
LARGE{Additional pages with vertical bar}
end{center}
lipsum[1-20]
restoregeometry
%clearpage

% Dummy code for last template page with horizontal bar
begin{tikzpicture}[remember picture,overlay]%
fill[gray] (current page.north west) rectangle %
++(paperwidth,-6.0cm);%
end{tikzpicture}%
vspace*{fill}
begin{center}
LARGE{Template pages with horizontal bar}
end{center}
vspace*{fill}

end{document}

documentclass[a4paper]{article}

usepackage[margin=1.0cm,noheadfoot]{geometry}
usepackage{atbegshi,tikz,lipsum,ifthen}
pagestyle{empty}

newboolean{drawSidebar}
setboolean{drawSidebar}{false}
AtBeginShipout{ifthenelse{boolean{drawSidebar}}{
begin{tikzpicture}[remember picture,overlay]%
fill[gray] (current page.north west) rectangle %
++(5.0cm,-paperheight);%
end{tikzpicture}%
}{}}

begin{document}

% Dummy code for template page with vertical bar
begin{tikzpicture}[remember picture,overlay]%
fill[gray] (current page.north west) rectangle %
++(5.0cm,-paperheight);%
end{tikzpicture}%
vspace*{fill}
begin{center}
LARGE{Template page with vertical bar}
end{center}
vspace*{fill}
%clearpage

% Additional pages with sidebar and dummy text
setboolean{drawSidebar}{true}
newgeometry{left=6.0cm,right=1.0cm,top=1.0cm,bottom=1.0cm,noheadfoot}
begin{center}
LARGE{Additional pages with vertical bar}
end{center}
lipsum[1-20]
setboolean{drawSidebar}{false}
restoregeometry
%clearpage

% Dummy code for template page with horizontal bar
begin{tikzpicture}[remember picture,overlay]%
fill[gray] (current page.north west) rectangle %
++(paperwidth,-6.0cm);%
end{tikzpicture}%
vspace*{fill}
begin{center}
LARGE{Template pages with horizontal bar}
end{center}
vspace*{fill}

end{document}