It's a witch's brew mostly. For the sepia brown look, many people use a lot of burnt umber, but normally, there are other colors in there as well in smaller amounts.

Actually, I've found that BU looks very different from different paint manufacturers, so it does depend somewhat on the paint you choose to use.

Colors I use a good bit are:
burnt umber
sepia brown
ivory black
windsor blue
indian yellow

There's many others, and depending on the manufacturer, they may have different names and slightly different effects.

The process of making a combination gum bichromate and palladium print is iterative, and sometimes when I'm trying out new colors, it doesn't work out, and the print goes into the circular file.

My prints typically have two or three gum layers over the palladium base print. The darker ones often have four layers of gum, and lighter ones may only have one.