documentclass{report}
usepackage{tikz}
pgfdeclareverticalshading{titlepage}{paperwidth+3cm}
 {color(0cm)=(red); color(0.5cm)=(blue); color(3cm)=(yellow)}

begin{document}
begin{tikzpicture}[overlay,remember picture]

shade[shading=titlepage,
       shading angle=15 %?????
       ]([yshift=4cm]current page.west)--++(15:1.2paperwidth)--++(0,-2cm) --([yshift=2cm]current page.west)--cycle;

shade[shading=axis,
       top color=yellow,
       bottom color=blue,
       %middle color=red,
       shading angle=30 %????
       ](current page.west)--++(15:1.2paperwidth)--++(0,-2cm) --([yshift=-2cm]current page.west)--cycle;
end{tikzpicture}
mbox{}
end{document}

documentclass{report}
usepackage{tikz}
pgfdeclareverticalshading{titlepage}{3cm}
 {color(0cm)=(red); color(0.5cm)=(blue); color(3cm)=(yellow)}
newsaveboxUlrikeShading
newsaveboxUlrikeShadingB
begin{document}
saveboxUlrikeShading{%
tikz{shade[shading=titlepage] (0,0) rectangle (2paperwidth+3cm,2cm);}}
saveboxUlrikeShadingB{%
tikz{shade[shading=axis,
       top color=yellow,
       bottom color=blue] (0,0) rectangle (2paperwidth+3cm,2cm);}}
begin{tikzpicture}[overlay,remember picture]
begin{scope}
clip      ([yshift=4cm]current page.west)--++(15:1.2paperwidth)--++(0,-2cm) --([yshift=2cm]current page.west)--cycle;
path ([yshift=3cm]current page.west) +(15:0.6paperwidth)
node[rotate=15] {useboxUlrikeShading};
end{scope}
begin{scope}
clip(current page.west)--++(15:1.2paperwidth)--++(0,-2cm) --([yshift=-2cm]current page.west)--cycle;
path ([yshift=-1cm]current page.west) +(15:0.6paperwidth)
node[rotate=15] {useboxUlrikeShadingB};
end{scope}
end{tikzpicture}
mbox{}
end{document}

documentclass{report}
usepackage{tikz}
pagestyle{empty}
begin{document}
begin{tikzpicture}[overlay,remember picture]
shade[shading=axis,
       top color=yellow,
       bottom color=blue,
       %middle color=red,
       shading angle=30 %????
       ](current page.west)--++(15:1.2paperwidth)--++(0,-2cm) --([yshift=-2cm]current page.west)--cycle;

shade[shading=axis,
       top color=yellow,
       bottom color=blue,
       %middle color=red,
       shading angle=30]
([yshift={2cm+sin(15)*1.2paperwidth}]current page.south west)
rectangle (current page.south east);
draw ([yshift=2cm]current page.south west)
--++(15:1.2paperwidth)--++(0,-2cm) --(current page.south west)--cycle;
end{tikzpicture}
mbox{}
end{document}