The leaves of 2 of my nephthytis plants are lightening and then dying. Why is this happening?
There are a lot of possibilities, chief among them is over-watering. Other possibilities are nutritional issues (deficiencies/toxicities), being root bound, soil compaction, a high level of soluble salts in the soil, a recent sudden reduction in light exposure, insect predation, disease ..... I'm sorry to say there is no way to offer meaningful guidance w/o more information.
There are two ways to look at arising issues. One is to address them as they rear their head; the other is to concentrate on providing the cultural conditions the plant prefers and reacts favorably to, so you don't have to address them. The later is certainly the better way to approach growing - sort of like preventive maintenance can circumvent larger headaches down the pike.
Something more accurate might be offered in help if you wanted to supply some more background. Meanwhile, familiarizing yourself with some of the basics might go a long way toward your becoming vested with the ability to self-review some f your habits. The link below might offer some clues.
Here is a link that might be useful: Basic growing practices