From above: I don't understand what makes their line numbers pink background instead of yellow. Is yellow used for the first line of difference?

Answer: The currently selected change is highlighted with yellow on the line numbers. If you turn on the toolbar you can navigate between changes.

From above: Then I get a line number with white background and black digits. I don't see anything different in the line in either window, so I don't know why it is there.

Answer: If it is black it is unchanged. The reason it shows up is probaly because it is directly between two changes, e.g. if line 5 and 7 had changes, line 6 will show up in black. This is a bug in the code that controls what is skipped in "context" or  "diff only" (which is just zero context).

From above: Finally, I see the last line is a white background, black didgits, and no coloring within the lines.

Answer: I'm guessing this is related to the same bug as above. An extra line slips through after the last change.