Hello and welcome!
When it comes to Amazons, I am not 100% sure, but I do have some ideas(having known people that have bred them).
I am not sure if it is a simple recessive issue like amelanism, but it is a fairly simple idea.
Let's go with your pair of ammies. You have a yellow sire and a garden phase dam. As you said, chances are pretty good that you get around a 50/50 ratio of yellow to garden phase babies. BUT, you could also end up with a red baby, or an orange, or ALL garden phase or ALL yellow, or any mix of color phases really. I think it all boils down to the pedigree of the parents. I have seen a litter from two red Amazons that was half candycane(red and white), with the majority of the rest being red or red with orange markings, and one solid yellow baby. The parents were both red for the most part, solid red(except a few black flecks here and there). But yet, the babies were an odd mix of colors. Perhaps one of the parents had a yellow sire or dam and that threw in the possibility of yellow being produced.
I think that is half the fun with polymorphic species like Amazons, you can make all kinds of guesses and predictions, but in the end you never really know what you are going to end up with until the babies are born