Starting last year (2003) Green stickers were issued for 2001 or older bikes. I heard there was some kind of mix up and 2002 bikes might get them also.
And just to clarify, the green/red sticker is for OHV areas only (public land desingnated for off-road vehicle use). You can ride all year on any bike you want as long as it's at a track and you pay the gate fee.
I think the green/red sticker program was developed for pollution control. If a manufactured off-road bike meets certain emission regulations it will get a green sticker, otherwise, red sticker.