Hiya! Sorry! I am very late for Charlies Linen Backing party.
So I had a look at some of the images and here's what I see.
Of course these are friendly pointers!
1. You want as much natural day light as possible. It looks like you guys are working in the evening with a single light?
If schedule allows work in day time? open the curtains flood the room with as much natural light as possible for as accurate match as possible.
Two! right, a bit dark in the room, but I am trying to make out what you are using for touch ups. My guess is Caran d'Ache Neocolor II? If so, these are very soft/creamy/Oil based. They won dry, they will smudge any time, even few years down the road. They are reversible, but they most likely will leave oil stains in the tracks after removing. Also they are not really fine point. You want something with a super fine point for better detail and less intrusive fold touch ups.
I use Faber-Castell Albrecht Durer Watercolor Pencils.
Anyhow, I am starting to feel like a party pooper. Maybe this should be my last post.
All the best,
dario.