I always think of grey genes as two layers:)
A chesnut has two brown layers.
A rose grey has a grey layer and a brown layer.
A grey has two grey layers.
A normal grey will always produce a grey. A rose grey has a 50/50 chance, like they are half and half chesnut/grey:)
I have a rose grey filly, she was born a cute dark chestnut
Then I have a mare who WAS a rose grey, and is now just a normal fleabitten grey. That's where she got her name Rosie:)