Let’s apply a little common sense here: “Is unused brake fluid in the container still good a few months later?” It probably is if the cap was sealed airtight. Your brake system is sealed too.
Brake fluid (DOT 3 and 4) is hydroscopic meaning it absorbs moisture. It also absorbs other gasses too. Moisture (water) is the most damaging contaminate to your brake system because it is corrosive and will eventually pit the metal surfaces of the piston walls leading to leakage, so that is one reason brake fluid should be changed. Another reason is it turns brown and begins to crystallize. It crystallizes because it is nearing its maximum limit of absorbing water and other gasses. These crystals cause clogging of the small passages in the master cylinder, most commonly the return vent which is the smallest being about diameter of a pin or smaller. In the worst cases it clogs up everything and you may end up replacing the lines and components. The best practice is to change your brake fluid often enough to avoid this kind of damage. I like to change my brake fluid once a year on all my vehicles.
New brake fluid is dehydrated. As the brake fluid goes through its service life, it begins to absorb contaminates (water and other gasses) turning brown as it reaches its hydration limit. Eventually it reaches a saturation point and the corrosive effects go to work. By then it is long overdue for a change. Pitting and the crystallization are the bad things. The brown color means it needs to be changed and is long overdue.
It is a good thing that brake fluid is hydroscopic and does absorb water and gasses. In a sense the gasses and moisture are neutralized and suspended in the fluid so the harmful effects are delayed. Over time the absorption limit is reached and the excess contaminates are free to do their damage.
The best practice is to use new brake fluid after it has been opened. Once it has been opened it has been exposed to the air and has started absorbing moisture like a dry sponge. I have never seen brake fluid turn brown in the can and have wondered why is this clear fluid considered bad? It’s probably not bad at all, just compromised some from exposure to the air while it was opened or a leaky cap. I would be inclined to use it in a pinch, but would be mindful that the system needs a fluid change soon. Relatively speaking brake fluid is cheap, so start with new brake fluid. Also flush the system if you can – that is the best practice.
Since brake fluid is supposed to be dehydrated from the manufacture, I would imagine that brand new brake fluid varies from one quality control standard to the next. You could possibly be adding new fluid that has more moisture in it than an opened can of fluid in the garage. Both look clear, both are still serviceable. You just do not have a meaningful way to know how much moisture has been absorbed. I have not found any products that are available to measure contamination of brake fluid, so the best practice is to use new fluid. The only way to know is by the shades of the brown color or if it looks cloudy. It’s cheap, so add or start with new fluid.
This discussion was about brake fluid absorbing air and moisture. Gasses enter fluids under pressure and exit when pressure is relived. Unwanted gasses and excess moisture in a brake system are undesirable because excess gasses lead to spongy brakes or clutches and moisture to corrosion. Generally speaking new brake fluid is more ideal than unused brake fluid because it is dehydrated and has the fewest contaminates. Less contaminates means fewer unwanted gasses entering and exiting the fluid while in service. As it gets older it absorbs contaminates like a sponge. Eventually the fluid like the sponge becomes saturated and will hold no more. To avoid saturation change your brake fluid often and the costly repairs.
I hope this helps.