Natural sunlight, a healthy balanced diet of fresh vegetables and fruit, plenty of sleep, and the use of pellets to help with mineral and vitamin balance will allow for naturally beautiful, vibrant feather colouration. Nothing can substitute natural sunlight, and your parrots need it for producing vitamin D, which in turn produces brilliant feather colours.
I'm no expert, but I do know that a combination of the above allows for healthy feathers. Sleep is also very important, just like it is for you. Your parrot should be receiving around 10-14 hours of uninterrupted sleep in a quiet, dark space. These changes won't happen within a day, but if you implement healthy food with a healthy environment, your IRN's feathers should become much better after their molt.
Here are some websites that give the important information regarding issues with feather colouration, how to help with feather coloration, and what your parrot needs to be healthy:
http://www.justanswer.com/bird-vet/6d5zx-green-indian-ring-neck-parakeet-feathers-turning-brown.html
http://birds.about.com/od/birdhealth/tp/Bird-Feather-Health.htm
http://www.drsfostersmith.com/pic/article.cfm?aid=1012
http://www.doityourself.com/stry/best-type-of-pet-bird-food-to-improve-feathers#.Uv0bNJe9KK0
I also recommend going to
www.indianringneck.com and consulting more IRN owners on that website. It's very useful and many of the members of the forums are extremely helpful.
EDIT: I just want to mention that ultimately, you do need to see an avian veterinarian to get to the bottom of the issue. We can only advise so much and give so much useful information, but at the end of the day a professionally trained avian vet will, most of the time, know the actual cause and will be able to give you appropriate diagnosis. Please do try to find a vet nearby, even if it's a long drive. Nothing should stop you from trying to provide the healthiest, happiest life that your new fid deserves.