Debbie, I think everyone realizes you were merely trying to be helpful by offering what is, frankly, a very logical explanation for the color change.
I'm sure that something in the diet very often is a key cause behind such changes in some birds. Yet I believe we can most definitely rule that out in this specific case.

As Janine has attested, sometimes colors do shift a bit with the 2nd molt -- apparently some birds do not completely achieve their full adult head hues until that point. This probably happens only rarely, and may well be genetic in origin. Just one of those things we'll never know why.
I'm very grateful for your input and for the equally unique insight everyone else has provided, as well. That's why I'm here (and I suspect is why each of us is here). To learn, and to share. I have nothing but the utmost respect for you, Tiffany, Candice, Liz, Sally, Janine and all the numerous other seasoned finchers on here -- I honestly would have been lost many times without their guidance these past 8 months.
I also realize that no one among us -- however new or however seasoned -- is wholly immune to making mistakes now and then. It's part of being human. We're all just doing the best we can. I don't look at this forum as the ultimate, fail-proof authority for finch-keeping -- because that doesn't exist anywhere. Rather, I consider it a welcoming, friendly and respectful community forum where each of us learns from one another. By pooling our collective insight, experiences, and ideas, we all benefit. That's the beauty of a forum such as this. Any given person at times will be the seeker of an answer. Other times that very same person will be the provider of an answer to another's question. And as in life, no single one of us has ALL the answers, or is right ALL the time.
Yet obviously, not all insight is created equal -- experience adds credibility. As a novice fincher, I'm still earning my wings in this department. So I mostly try to just "listen and learn" for the time being -- and remain fully receptive to any and all ideas offered. I had invested collectively several weeks of time (a few minutes of spare time here, a half-hour or hour there, etc.) to becoming an informed bird parent beginning in February of last year, before finally picking up my birds in October. Eight months of self-imposed finch "boot camp".
I wanted to be as informed as possible, so I read a book on Gouldians from cover to cover -- twice. I read another book on keeping finches. I pored over often conflicting online data on how to do things, including more than a week just trying to sort out the best possible lighting options -- even asking several avian vets for their advice. (Hey, it's the old news reporter in me!) And of course I monitored this forum throughout the 8 months whenever I had time. There were times when these new responsibilities I was preparing to assume seemed overwhelming and hopelessly complicated. I recall almost backing out at one point, when I couldn't find a clear-cut answer on the seed vs. pellets debate! (I ultimately resolved this by deciding to offer them both.) Bottom line: I did my homework.
But as I recently said to Tiffany -- no matter how well prepared you THINK you are before you add birds to your home, some things must be learned only through firsthand experience. After all, as we all know, birds at times develop in ways that simply don't follow the rulebooks! There also are just countless situations bound to arise that were never covered at all in your preparatory research, or if so, then in insufficient detail for you to be able to adequately handle it without the insightful help of others. This forum is absolutely invaluable in that regard.
And for every finch whose life has been saved -- or improved -- because of advice proffered here, this forum is priceless.
I know for a fact that my own birds' lives are improved because of this forum and the compassionate dedication of its members. (And I suspect that Tiffany's life has improved as well, because it allows me to avoid pestering her every time I'm fretting over some new development!

) For the numerous nuggets of wisdom offered to me over the last several months, I offer a genuine, heartfelt THANK YOU. I sincerely hope that one day I can feel as confident in my own finching abilities to be as helpful when needed.