psylxa[X_] :=
     {ColorConvert[
           Hue[Arg[X]/(2*Pi), 1,
             1], "RGB"][[1]],
       ColorConvert[
           Hue[Arg[X]/(2*Pi), 1,
             1], "RGB"][[2]],
       ColorConvert[
           Hue[Arg[X]/(2*Pi), 1,
             1], "RGB"][[3]]};
tsylxe[X_] := RGBColor[
       X[[1]], X[[2]], X[[3]]];

Show[Table[ParametricPlot3D[
       {(c + Cos[t])*Cos[a*t],
         (c + Cos[t])*Sin[a*t],
         Sin[t]}, {t, -4*r*Pi +
           (8*Pi*r*n)/20, -4*r*Pi +
           (8*Pi*r*(n + 1))/20},
       PlotRange -> All,
       Boxed -> False, Mesh ->
         None, PlotStyle ->
         Directive[Opacity[0.4],
           tsylxe[psylxa[
               Exp[n*I]]]]],
     {n, 1, 20}]]