The ZX-6 (as opposed to the ZX-6R) went from 1992 to 2002 virtually unchanged, which makes it very difficult to pinpoint years.
Probably worth giving Kawasaki a call with the frame # & see if they can confirm the year. They are quite helpful on stuff like that in the UK so I would hope that they are the same in the USA. Good luck.
I have a 2002 that you can see in my ICON. The frame is black powder coated. I thank that Kawasaki started the black frame in 2001. I don’t know for sure but the bike in your picture could be a 99 or 2000. Check the serial number. It can be found on the right side of the steering stem right under the top triple clamp.
The tenth digit of the vin is the manufacturer's model year. Starting in 2000, they are 0 (2000), 1 (2001), 2 (2002) etc. I think they ended near W, X before the change-over, which would be a late 90's model.