I need to go back through and recategorize some of the cancel color assignments, as many were done early on in my revenue collecting. In my opinion, ink color on manuscript cancels is pretty much immaterial to value (with one exception, which I will get to later). Assigning colors is really only useful and contribute to value on handstamped, printed, and stencilled cancels.
True brown ink cancels, as opposed to faded or otherwise environmentally affected black/gray ink, are very scarce... more scarce than green, which is also quite scarce, although less so.
One needs to be careful when calling a cancel green, as it frequently is actually blue, but when applied to yellow or brown stamps can cause the optical illusion of green.
The most scarce cancel ink color, in my opinion, is gold or other metallic ink. There was an ornate gold-ink handstamp cancel in the Cunliffe collection, if memory serves. That is the only gold handstamp I have ever seen. I have two examples of manuscript cancels that I believe to be gold ink.
Here are some examples of what I consider to be brown ink cancels, the first one being a stencil used as a photographer's backstamp rather than an actual stamp cancel in this case:





Green:




The two gold ink cancels:

