I'm assuming you've already counted out the K2 Extreme which was an early 90s release from K2 w/ the multicolored bottoms. I'm trying to think back about all the K2 skis that I sold at that time & can't come up w/ a black top, green bottom ski. Any idea if it was a high end ski, intermediate ski, etc. Why did you buy it? Did you say you wanted a bump ski, slalom, GS, etc. that will provide some guidance and help narrow down the potential candidates. If it was an intermediate or lower end ski it could be a unique graphic that was created for a certain market or store. It was quite common for them at the time to make multiple graphics. Many of the large retailers like Big 5 for example would have a unique ski that was exactly the same as a 5500 for example or TNS that they sold at a steep discount. Any specialty ski store couldn't touch the price, but if asked to match it was an easy reply of No, it's not the same ski. If it wasn't a US ski, it could be even tougher to track down.
From memory popular K2 skis of early 90s:
TNC - black
Extreme - black
5500 - black
TRC - believe they had some in black
Gyrator - pre Extreme was black
They also had the Grateful Dead ski, but would assume you'd have not overlooked that graphic.