Turnip
Regarding "You can't get blood from a stone."; I had always heard the statement "you can't get blood from a turnip." It has the same meaning, but it at least starts out with the same or similar color.
Both phrases are in use. I did a Google war using the exact words, and it was a virtual tie, 940 to 887.
Similar color? Weren't you thinking of beets? The turnips I know are light brown.