DogsNDaylilies said:@admmad - Maurice, you don't happen to know the answer to this, do you? You're very knowledgeable with genetics in general...
I searched for any research that looked at the flower colours in Hippeastrum and unfortunately there is very little and it is not known why there might be more pigment on one side of the flowers versus the other.
I can make two guesses; they would not be mutually exclusive.
One is that in the bud the flower parts might be more or less stacked on top of each other with sepals covering petals and parts of upper petals overlapping and covering parts of lower petals (etc.).
The other is that the buds might be more or less arranged horizontally rather than vertically while they develop. The flower parts in the top half of the bud would "cover" or shade the flower parts in the bottom half of the bud.
If either or both of these is the case then since anthocyanin pigments are strongly affected by both temperature and the amount of light and the "lower" petal would be shaded/shadowed by the upper petal and sepals it might develop less anthocyanin pigment
