Indeed, it should read either dark blue or Prussian blue. No field grey before the C20th.

Mmm...looks like someone was having a brain fart on that day.  'Someone' will go and fix that error....

Out of interest, Bob Marrion's book on Prussian uniforms of the FPW has now been joined by his book on the French uniforms. This is very late to the party mainly because it was unfinished. It has been edited and presumably completed by Stephen Shann. Both are available from Caliver Books. I am currently awaiting delivery of a French book on the French uniforms of the FPW and will post a review when it arrives.