Here's what I've learned for what it's worth. When I got Kili from a bird store, her plumage was not good at all. Her tail feathers were all torn up and she had endless stress bars. However, with good diet, exercise, and out door time she has developed a stunning plumage. Here are before/after photos from a previous molt. I think she looks even better now so I'll have to find a more recent pic to post.


In terms of diet during molt, I seriously think it's better to feed a close to 100% pellet diet during this time assuming it's a top quality pellet. Almost any other foods will dilute the nutrition. I agree with feeding more variety between molts but feeding about 80%+ pellets, 10% nuts/seeds as treats, and 10% other has definitely yielded some good results. Truman's breeder and vet agree with this approach so I'm not just making it up.
Second, I think out of cage time and flight help the overall health/fitness of the parrot and this is reflected in the plumage. In nature the parrot with the sexiest plumage gets the hottest chicks, right? This is probably a selection for parrot parents that have better nutrition/health which ensures survivability of their offspring.
Finally, I don't think any kind of lamp or indoor setup can replace the value of outdoor sunlight. I don't know if or how much of a role this plays but to me it seems that it does. Since I've started taking Kili out, her eye color has become brighter and I think it is reflected in the plumage as well. However, I cannot be certain what kind of role this plays compared to the two above factors because I had begun experimenting with all 3 of them around the same time. So which of the three plays the biggest role I cannot tell you, but to me it seems that since implementing the three it has done wonders on her plumage.