Yep, seeds and millet are birdie "junk foods" feeding only these would be a human equivalent of feeding children nothing but McDonald's food everyday, every meal. That being said, just like at McDonald's, there are some healthier seeds for birds - Safflower and Flax seeds come to mind instantly without any other research (I'm feeling lazy and don't want to open another tab and google anything right now ... ) These seeds are lower in the "bad fats" and have a lower caloric value, but again these should just be supplements to a healthy diet of fruits and veggies (stay away from the normal "offenders" when it comes to bad veggies ... if unsure which these are please ask) and a good pellet based diet (I am personally in favor of Zupreem and Harrisons, but that's just my opinion as formed by my avian vet ... ). Green leafy veggies (Collard Greens, Mustard Greens, Kale ... avoid spinich) high in beta carotine will make for magnificent adult colors (it makes a hugh difference, their colors just *POP* if they've been feed a proper diet)
Helpful hint to all - My wife and I have started buying premixed "stir-fry" veggies for like $2.99 a bag (find them in the pre-bagged salad sections of your grocery/market) the bag usually contains brocolli (precut) carrots (precut) and snap peas. I then grab a few loose string beans and some sort of "leafy" veggie mention above ... my girls get fresh veggies all week long and it only cost me like $4 a week, and takes me like 10 minutes to chop everything when we get home from the grocery ... those supplement a pellet based diet and we have no stress bars, and no other heath issues (according to our vet) ... it is neither costly nor time consuming to make sure that our birds get a healthy diet ...
As for fruits ... they eat what we eat... peaches, plums, bananas, oranges we will either cut some up for them, or if wer forget, they will come over and help themselves to what we are eating ..
Again, it's easy ... and you will have healthier, happier parrots in the long run ...