Yellow leaves can be a sign of virtually anything--bright light, overwatering, underwatering, nutrient deficiency, pathogens, non-optimal humidity, too hot, too cold....etc.
But, with that said, I grow all my neps under fluorescent lights and some develop yellowish (and sometimes reddish) leaves. In my case, it's probably the light because I foliar feed with an orchid fertilizer once per month, and my plants are growing and pitchering like mad so I believe they're well fed.
Why don't you try foliar feeding once per month with an orchid/epiphyte fertilizer diluted to 1/4 the recommended strength? It's pretty safe with neps and I would recommend you place your plants in lower light while they're absorbing the liquid as some species can have their leaves slighly burned if placed under bright light with lots of water on them.