Hey Blackbeard,
The short of it is this: After finishing up my project, whatever it is, I hand-sand most of the black off so I get a lot of base metal showing, then I just reheat to the point of getting the oxidation colors. With small things like the bowls I just put them in the forge for a little bit. The colors in the line gradient is because the end was in the flame path. With the pot rack I just clamped it in my vise and heated it with a propane torch until the color flowed. If you work with coal or charcoal, you could just hold the object above the flame and watch for the colors. as soon as I get what I want, I quick dip or pour water over it to stop the colors and rub it down with beeswax.
If I have a lot of small things like crosses or keychain leafs, I clean them all up and put them in my forge after I have turned it off.