Does it need to be in a dark color glass bottle or does it matter.

Doesn't matter. I keep mine in a clear Boston bottle with a hard plastic cap. A metal cap will rust away in a few months as the quill is very costic. Quill will not freeze so you can store it in an out building. I keep mine in an old refrigerator in a shed.

How long can it be stored and still be useful?
Have some i put in a Boston btl then into an ice cream bucket that I buried in the garden, I moved & haven't grabbed it yet. That was bout 3 yrs ago.

I have had gallons stored for 20 plus years. Use it every year. Keep your lids secure and it will hold a long time. You may lose a bit over the years due to evaporation just the nature of the beast.

store my skunk essence in 1 oz or 4 oz glass bottles, sealing each with either a wax seal or duct tape around the lid to help contain odor. The bottles are then placed inside a metal ammo can with pine shavings, which can absorb minor leaks and provide an added layer of odor control. I would just avoid storing in direct sunlight so you don't lose product due to evaporation, but that's typically not an issue if you keep a good lid seal.

As an essence buyer I prefer it in bigger bottles. For example if you have a pint I prefer to buy in a pint bottle vs 16 1 oz bottles.

Also, if you store in a bottle with a known volume it is easier for the buyer to price your essence.