It’s red.
http://my.clevelandclinic.org/health/articles/22114-red-light-therapy
The link has what appears to be good information about the red LED therapy. You can even purchase your own device for that part of the treatment. All that I’m reading about Dupixent and the light therapy points to eczema. Did the doctor give you a specific diagnosis or is this all to eliminate possibilities while treating the symptoms? Obviously, it’s none of my business, but you and your wife need to know what’s going on before making decisions about what looks like an extremely expensive drug.
There are alternatives to Dupixent noted at http://druggenius.com/alternatives/dupixent/ , but it might not hurt (too much in the bank account) to get a second opinion.
I finally found the cost of Dupixent, at least for the uninsured. As one of those, I would have to forego it and try the OTC ointments and lotions and maybe try out the red LED therapy in some form. A variety of devices are available on Amazon and probably elsewhere. Some of them look very targeted and some are in lamp form. Either way, you have to block the light from your eyes and make sure any medications you’re taking are compatible.
May God bless her with complete healing!