I use empty soda bottles as the plastic doesn't react with black and white developers, stop bath and fixer. The cap seals the bottle completely and works over and over. I imagine this is because of the fizz in soda, designed so it doesn't go flat. I use empty 1 liter bottles for Perceptol. You'll find the stuff lasts a long time if stored in a cool place that's dark. For me, that's using a cabinet in the darkroom.
I believe the dark stuff you see are chemicals reacting with oxygen along with heat and light.
I do have some Pyro developers but I keep the liquid in the plasic containers they came in from formulary.
Hope this helps you.
I make photographs as a return ticket to a moment otherwise gone.