The 2 transistor limiter and the LM317 limiter are the simplest. The LM317 is simpler and with one of those little blue 25 turn trimmer potentiometers, you can set the thing to anything you want.
You have to be careful though. The LM317 in the TO220 package has a low end current limiting limit of about 10ma. You might be able to push it a bit lower but its not guaranteed.
The LM317LZ (TO92 package) is guaranteed down to 2.5ma I believe and its plenty good enough for a Colloidal Silver current limiter even up to say 20-25ma.
For safety's sake you should put about a 50 Ohm resistor in series with your potentiometer. That'll limit the highest current to about 25ma. The reason being is that those little blue 25 turn trimmer pots can achieve damn near zero resistance at the low end and that would max out the regulator for an LM317LZ (100ma max) and its a safety thing because, unless you're making bathtubs full of the stuff, 25ma is on the high side for most people. On the low end, a 1K blue trimmer pot will allow you to set it down to as low as 1.25ma BUT with the LM317LZ I wouldn't expect to see much below about the 2.5ma guarantee (maybe 2 but much below this I'd be surprised).