I saw a carbon steel knife a Sharpfan used catsup to color it. The nice had a factory etch on the blade and the catsup color treatment enhanced and complimented it beautifully. I will try to remember who it was and get a pic.
An interesting topic for sure.
There are a ton of methods to be used, it all just depends on what color you are trying to acquire.
I have seen a solid black 1095 blade, all thanks to some boiling white vinegar.
I use vinegar when forcing a patina and it doesn't take that much to do. I like a light grey to medium patina on my work knives, so I just put a little vinegar in the microwave and I give it some warmth. I use a q-tip to apply and it seems to work just fine. I give it a rain drop affect by leaving drops of vinegar sit on the blade. The warmer, or longer the liquid is, or left on the blade the darker the patina gets.
I will go over the blade with a little oil and some light steel wool after the vinegar treatments. When it's at a look that I want, I oil it up and leave it sit a day or two. You can do a solid patina the same way.
Here is one I did a vinegar treatment to.