There are a couple of ways I tell my greys apart.
First and foremost in the beginning, Dot was bigger than Fiona Even though they are the same age.
Second, Dot has really big ears compared to Fiona. She has a confidence about her and holds her ears up on and is alert most of the time where Fiona isn't as confident and her ears are usually partly folded.
I had to take a pic of them side by side and I noticed their head stripe touches Fiona's nose and Dot's doesn't.
And last but not least, their personality. Dot is quick to come forward for treats, where Fiona hesitates.
Fiona's coat is a little darker too. But in the dark, I looks at ears and nose stripe.
When I reach in their pouch, Dot is on top, but easily dives to the bottom so I bring Fiona out first most of the time. I can tell that by her size.
Now Stewie is a mosaic color and he has a white tail, legs and belly. He practically glows in the dark compared to the girls. He is in a separate cage untill he grows a little more.
I hope this helps.