I have a "G" (med. green) for Hasselblad here that is marked 3x/-1.5
I have a "YG" (light yellow greenish) that is marked 2x/-1
I always remembered the dark green ones for foliage were close to 2.5-3 stops but haven't used one in years.
If no one knows exactly I can try to dig up a wratten series 6 I have somewhere but keep in mind if it's a gelatin type they can fade. Best thing to do is test and hold it over your handheld meter and compare against unmetered.