Do not pour unused blue solution (any type of blueing solution - cold blue, rust blue, etc) back into the bottle.
Do not apply cold or rust blue solutions from the bottle. Pour some into a small container and apply from there.
If you pour back unused solution or apply by dipping your application q-tip, pad, etc into the main bottle, you'll contaminate the rest of your blueing product.
There are alternatives for cold blueing to Birchwood Casey or Hoppes. Look in Brownells for additional products, and Caswell Plating for other bulk product.
If you're really interested in making your own blueing solutions, the book to get is "Firearm Blueing and Browning," by R.H. Angier. You can get it in a bunch of places on the 'net, or directly from Stackpole Books, who have a bunch of gun books.
Angier's book was written in the 30's, and uses some archaic chemical names, which you might have to 'translate' into modern chemistry. Some of the chemicals used in Angier's book aren't exactly friendly stuff - like cold, fuming nitric acid, or potassium ferricyanide, picric acid, etc.