I have been using the 2 oz bottles of cold gun blue sold for use in touching up firearms. Works OK.
As someone has already noted, these are more effective on some steels than on others and of course not on stainless at all (unless specifically for stainless).
These solutions are acidic in nature and you need to degrease your piece very well before starting.
What I do:
1) Degrease part - use your favourite method but get it clean.
2) pour the contents of the 2oz bottle into a plastic cup and place the item to be blued into the solution.
3) swish, swirl, stir or whatever until desired color is attained.
4) remove and wipe with clean rag
5) place freshly blued item into another plastic cup containing a week alkylii - I use something like spray 409 or simlar household cleaner and swish swirl or whatever for a bit. This step neutralizes the acid of the bluing solution. Simply wiping, washing and oiling isn't sufficient - several piecies I have blued without this step but only washed and oiled ended up showing rust in threaded areas and corners and edges. I haven't had this happen since I have started using the neutralizing step. **note: when you place your part in the alkylii solution you may get some bubbling which should only be momentary.
6) If I don't need or require the part immediately I will put it in a container with enough oil to cover the part until I need it, sometimes for an hour sometimes over night - this step is probably not needed if you give the part a good and thorough oiling. This is just my work flow - the last thing I do is blue the part, set it to soak, clean up and turn out the lights.
7) the bluing solution in the first plastic cup is then put back in its original bottle for use another day. Dabbing the solution on the part works as well but I found it quite wasteful for small parts as well as awkward for small parts. You will find the solution will last some time but you will get some precipitate in the bottle that is best filtered out the next time you pour it out of the bottle.
Gently warming the solution in warm water as well as you part will make the process more effective. But be aware that any kind of solution such as these that are warmed or heated will give of vapours that will be far more dangerous than if you had not heated the solution. Proceed cautiously!
Of course you mileage will vary.
You may find the information in this old book helpful:
see:
http://www.archive.org/details/metalcoloringfin00kauprich
This is a PDF downloadable document and quite interesting. It is dated in the early 1900's. some of the common names of the chemicals have changed over the years but a google search will sort that out. Many of the chemicals specificed are quite dangerous when improperly handled and used so do be careful.
What the world needs is a simple, safe, cold, metal coloring process to add blueig or browning or whatever that will provide a tough, hard wearing, long lasting finish to metal parts. So far there isn't one that I know of.
cheers, Graham