Although it first went through black cloth, the stain that got through to the white cloth cannot be dismissed as a stain on a colored surface.
If the size of the stain on the white cloth is clearly larger than a gris, it still may prove helpful to have the stain's color evaluated by a rabbi. Also, the status of synthetic fabrics with respect to stains is a matter of halachic dispute. If the white crotch was synthetic, that should be mentioned to the rabbi, as it may provide grounds for leniency.