Dies made in that manner didn't stay gray for very long. Vegtable dyes used in the Confederacy would oxydize very quickly turning brown or kaki in color. Most were made from Logwood or Sumac. I am sure there were other dye recipies. Ben Tart and a couple of other sutlers are making material in dye lots using natural dyes. Ben's web site is http://www.bentart.com/

May be some info there you could use.