I did a little bit of search about outfits in Naruto series, only 2 ninjas had a dark green outfit.
Shikaku and Asuma. If you check both of their profiles in Naruto wikia, nothing is being said about the color.
This is a peace from Asuma description:
His clothing consisted of the standard Konoha ninja uniform with the
sleeves rolled up half way, flak jacket, regular shinobi sandals and
forehead protector. He also wore a sash that had the kanji for "Fire"
(火) marked on it around his waist, a pair of black bangles, and
bandages wrapped around the arms of his sleeves.
Also the flak jacket, for Konohagakure it's only one, there is no other jacket that the Jōnins wear.
So I asume it has to do mostly with Shikaku that he was the Jōnin Commander, so we can understand the difference between him and the other Jōnins.
So to summarize, it's just a speculation about all this. But there are no evidence about these colours. Maybe in a Naruto SBS there is an explanation, about this, which I can't find.