Correct. You are talking about a gene pair at the “blue-not blue egg shell” location on the DNA. Since blue is dominant, all you need is one of the genes at that gene pair to be blue to get blue or green eggs. A true Ameraucana has two blue genes so he has to pass one of those to all his offspring. As long as he is truly an Ameraucana you are guaranteed to get pullets that lay blue or green eggs.
The exact shade of the blue or green shell is a lot more uncertain. There are a lot of different genes that influence the shade of the egg. Some of those genes are dominant, some recessive, some only act if another certain gene is present. Many chickens aren’t that pure for all those genes, especially if they lay brown or green eggs. So the shade you get can vary quite a bit even from full siblings, let alone half siblings.
You’d think that you would get something between the pure blue the rooster should have and whatever color the hen lays, and a lot of the time you do. But sometimes you can get some pretty big surprises. You have to hatch the eggs and raise the pullets to see what you get. But with a true Ameraucana they will be blue or green of some shade.
