Let me try to explain the underlying processes as far as cancer is concerned and the way it spread,
Cancer is a condition where the normal program of cell cycle/ regrowth is corrupted so the cell do not know when to stop making new cells, it like a computer virus you might had experienced it sometime while on internet, the browser keeps opening new windows on its own even though you are not asking to do any thing, its out of control , and you have no ways of stopping it. In as shame way the code for cell in written in DNA and the code can corrupted by various reason and can lead to what we call cancer.
Now the cancer are of two types 1) BENIGN (basically stays where it occurs no spread to other parts of body) and 2) MALIGNANT (tends to go to different places , mostly the cell travel through LYMPH
some of the malignant cell may travel to other organs via blood)
The kind of cancer in you mother in low was a MALIGNANT type where it got spread to the liver
, the other organs giving the worst out come, it just need only one cell to get to different part and will start reproducing itself and as we know from above explanation that the cell program is corrupted and no way to stop it giving a cancer node in place where it get lodged (in liver as mention by you)
Now the BROWN substance mostly will be the clot from the blood , some of the cancer tends to be "HEMORRHAGIC" - meaning will cause blood leakage around their area, and thats what probably you have seen.
Hope it helps, contact me in case of any other clarification is needed.