Maurice, thank you for your input! Well-presented, as always. The sunlight idea had occurred to me, but I didn't give it enough thought until you posed both of your ideas. It makes me think, now...
Considering the location that I have for my amaryllis indoors, I could see if the latter holds weight in my growing conditions. I've been keeping my amaryllis pots on the second floor of my home where they receive bright, indirect light with consistent temperature, meaning no sun to change the temperature from the top of the bloom to the bottom. And before you go getting all technical on me about how heat rises, let me state that I don't think 1/2" - 1" will make enough of a difference in my house to account for temperature-related pigment changes.
Unfortunately, the varieties I have all seem to be varieties without the "top-heavy" pigmentation, but if I acquire the Butterfly (Papilio) Amaryllis or one of the others that appear to have extra pigment on the top, I'll see what happens in my home.
