ff[x_] := Exp[x];

    
Plot[{ff[x], -ff[x], ff[-x], -ff[-x]}, {x, -3, 3}, 
      PlotStyle -> {{Thickness[0.008], Green}, {Thickness[0.008], 
            Blue}, {Thickness[0.008], Red}, {Thickness[0.008], Magenta}}, 
      PlotRange -> {{-3, 3}, {-6, 6}}]

ff[x_] := Exp[x];

    
Plot[{ff[x], ff[-x], ff[x] + ff[-x], 
        ff[x] - ff[-x]}, {x, -3, 3}, 
      PlotStyle -> {{Thickness[0.008], Green}, {Thickness[0.008], 
            Red}, {Thickness[0.008], Cyan}, {Thickness[0.008], Yellow}}, 
      PlotRange -> {{-3, 3}, {-6, 6}}]

ff[x_] := 
        x (x - 1);

Plot[{ff[x], -ff[x], ff[-x], -ff[-x]}, {x, -3, 3}, 
      PlotStyle -> {{Thickness[0.008], Green}, {Thickness[0.008], 
            Blue}, {Thickness[0.008], Red}, {Thickness[0.008], Magenta}}, 
      PlotRange -> {{-3, 3}, {-6, 6}}]