Robert Oster Signature Grün Schwarz or green-black ink is a very dark green almost black shade. The green dye tends to lean more blue than yellow so it’s more of a dark pine green colour.
In the pens, I used it had a tendency to become darker over a few days and it could sometimes be difficult to see much green at all in my writing. It’s a saturated ink and can exhibit some sheen depending on pen and paper used. With a finer nib you can see a little shading.
It was quite a wet ink to write with and had very good flow as well as very smooth lubrication across the page on my coated Clairefontaine Triomphe paper.
I didn’t notice any skips or hard starts when I put the uncapped pen down to do swab tests, dry times and water resistance. Nor did I see any feathering on the papers I wrote on.
Robert Oster inks come in 50ml clear plastic bottles with labels on top of the cap showing which ink is in the bottle. In my humble opinion, this is an idea that other ink manufacturers should copy.