JJ - it is indeed a very (very) dark green, usually referred to as "steel green." Like many of the more nuanced colors from the era, its hard to capture in photographs. (The cap is the same one pictured in that book, btw - p. 346)
Technically, it is the same green the Bavarian Chevauleger and Uhlans used, but in practice it always seemed to run darker. And add to that the slight fashion variations depending on the year...and you have a wider spectrum. Tony has a great example on his site of cornflower blue variations, which were all over the map!