Quick comments:
1) your definition of Graph
should not be a delayed evaluation (:=), it should be an immediate evaluation (=).
2) you should use graph
rather than Graph
. Graph
is a reserved Mathematica symbol. Generally it is a good idea to define your symbols starting with a lowercase letter to avoid this.
3) A safer and more powerful and unambiguous way to use PlotStyle
is with Directive
as in
graph = Plot[{x, x^2}, {x, -3, 3},
PlotStyle -> {Directive[Purple, Thick, Dashed], Directive[Pink, Thick, Opacity[0.5]]}]