Green gas is bad

What is green gas?
Green gas is the product name airsoft companies use for propane that is mixed with silicone oil. The idea behind this is that it removes much of the need for maintenance because the gun "lubricates itself" when the green gas passes through.

Why is green gas "bad"?
Green gas sprays silicone oil everywhere with every shot, meaning both that lots of areas that shouldn't have lubrication gets lubricated. This includes the nozzle, hop-up chamber and inner barrel which is places where you absolutely do not want any oil.

Oil not only attracts dirt and dust, it also messes with the hop-up bucking since at its core the hop-up relies on the friction of the bucking. (And if you use silicone buckings they can actually swell from silicone oil exposure.)

What about the other colors?
There are a couple of different mixes of propane and other gases marketed under different color names like Blue Gas, Red Gas and Black Gas with the purpose of controlling the pressure level.

They all contain silicone oil though unless they're explicitly sold as "dry" gasses.

Dry green gas
Some brands like Swiss Arms advertise "dry" variants of their green gas bottles where there is no silicone oil added. This is essentially just propane but without the smell additive, but can have a certain amount of a secondary gas to get the proper PSI level.

Propane
Depending on where you live[ref]In Europe it's most common to sell LPG rather than pure propane, so availability may vary.[/ref] and your tolerance smells using pure propane bottles is by far the best choice both from an economic perspective but also for the longevity/cleanliness of the internals. The go-to brand is often Coleman Propane but there are several brands of camping/barbeque bottles that are suitable.

Smell
For safety reasons propane companies add chemical called _mercaptan_ to give it a distinctive smell to easier identify leaks. While the chemical is harmless it has a distinct smell of rotten eggs that bothers some people.

Adapter
In order to fill your mags with pure propane you need an adapter that threads on the propane bottle with a standard fill nipple, for instance the MadBull XG