One way to look at it is that there are two base egg shell colors, blue or white. There is one gene pair that controls that. Since blue is dominant if just one of the genes at that pair is blue, the hen will lay a base blue egg. If you crack an egg and remove the membrane from the inside of the shell you will see the base color. Brown or green is made by putting brown on top of the base color.
Blue + no brown = blue
Blue + brown = green
White + no brown = white
White + brown = brown.
Brown comes in all kinds of different shades so you can get a huge range of different shades of green or brown shells.
I don't know the genetic make-up of your OE rooster. Roosters don't lay eggs so how can you tell unless you know the genetic make-up of both his parents? Since one of his daughters laid a brown egg there are two possibilities. In either case the Australorp/RIR hen contributes a not-blue gene.
Symbols sometimes help so in this large "O" stands for the dominant blue gene. A small "o" stands for the recessive not-blue gene.
If your male has one blue gene he is O,o and will pass one of these genes to his offspring at random. About half of his offspring should get an O and lay a base blue egg. The other half will get an o and lay a brown egg. Your hen will always give an o to all offspring.
The other possibility is that he does not have any blue genes. If that is the case none of the offspring will lay blue eggs.
If he were O,O then all of his offspring would get an O and all the girls would lay a base blue egg. That did not happen so he cannot be O,O.
Bottom line is that you will have to wait and see what the other pullets lay. I wish you luck.