Since there seems to have been a lot of confusion about differentiating the two species I'll lay out how to differentiate them .

Nigronia serricornis has a variable white band on the forewing, ranging from just a few white crossveins to a broad rectangular shape, there may be brown spots in the white costal area of the forewing but there typically will not be any brown spots in the rest of the white band, and the band will be opaque. The hindwing will have a strong to weak white band toward the apex and may have a weak white band toward the middle, and the rest of the hindwing will be a dark brown color. Will always have serrate antennae, deeper in males, shallower in females.

Nigronia fasciata has a consistent band on the forewing, it will always be broad and somewhat rectangular, it will typically have brown spots on the edge of the band closer to the base of the wing, and the band will be more translucent. The base of the hindwing will always be white, there will always be a strong white band toward the middle of the hindwing and there will usually be some white crossveins toward the apex of the hindwing. Females will have serrate antennae, males will have modified pectinate antennae.

Thanks for the info on how to differentiate the 2 species.
I've included this on the info page for the genus.