Currently in FilmPack 8, you need to select one of 13 presets or manually adjust the dedicated curves in order to remove the colour cast caused by the film mask.
When using NegativeLab Pro as a plugin to Lightroom Classic, the colour cast is removed simply by using the white balance eyedropper on the mask colour before inversion.
Please allow this approach in FilmPack, and in the associated PhotoLab panel.