That thing is a hydrometer, it measures specific gravity. That tells you the strength of the antifreeze, as far as what temperature it'll freeze. It tells you ABSOLUTELY NOTHING about whether the anti-corrosion chemicals are used up. The brown color tells you that you should've changed your antifreeze a long time ago.

Check your battery again (engine off) & turn on the lights & blowers & stuff. It shouldn't drop much below 12v.

Now run the engine, check the voltage at the battery. Running, it should be about 14 or 14.5v. Turn on all the electrical stuff you can, it should still stay about the same. This checks the alternator.