The Mathematica UI colourizes the FourierParameters in the following
Clear[x, y]
g[x_, y_] = x^2 y ;
f[x_, y_] =
FourierSeries[x^2 y, {x, y}, {4, 4}, FourierParameters -> {1, 2 Pi}]
as in the following
"Why color" on right click says it is excess arguments, but the resulting series computation matches nicely, and returns without error, so this seems like a UI colorizing bug to me.