A standalone sample illustrating an apparent front end colorization UI bug.

Clear[x, y]
g[x_, y_] = x^2 y ;

(* runs without error despite colorization: *)
f[x_, y_] =FourierSeries[x^2 y,{x,y},{4, 4},FourierParameters->{1, 2 Pi}]  ;
f[x, y] // TraditionalForm

(*$LicenseID*)

Plot3D[ g[x, y], {x, -2,2}, {y, -2, 2}, PlotRange->Full]
Plot3D[ Re[f[x, y]], {x, -2, 2}, {y, -2, 2}]

(* Observe that the Fourier fitting works quite nicely, even with only two terms *)
Plot3D[ {g[x, y],Re[f[x, y]]}, {x, -1/2, 1/2}, {y, -1/2, 1/2}, PlotRange -> Full]