I don't think that the colored lights of the Emotional Spectrum were supposed to be based on "true" electromagnetic radiation.
So, going as far as inventing an Ultraviolet Corps ... I don't know, it sounds, hmmm ... IMO, DC is pushing the concept too far.
If they wanted a 10th Corps, I had the idea of a Corps based on the Purple Light. It wouldn't be a new color since the Purple Boundary isn't a separate electromagnetic radiation but only how our mind pictures a combination of red & violet colors.
Thus, Purple would have been the perfect reverse Corps :
- Essentially, it would be the Emotional Spectrum without the Green Willpower.
- Red Rage & Violet Love would combine into a Purple Light which would be fueled by the same kind of zealotry than the one of Glorious Godfrey's Justifiers. It would be the equivalent of Anti-Life for the Emotional Spectrum.
Also, Purple Light already appeared in the Green Lantern book (it powered the Discs of the Corpse).
The members of this new Corps would have been the same kind of Qwardian enhanced soldiers than the Anti-Green Lantern Corps.
Contrary to the other Corps, they would look all alike.