Green chroma is not one specific color value. Once you project your slide, the exact color won't stay consistent anyway. The strength of the projection bulb and lighting conditions in the room (not to mention the color response of the camera you're shooting with) will change the resulting color you'll use for keying.
(I'm assuming that you're asking this because you are reshooting what will appear in front of the slide and will key it then. Your post was a little light on the full details of your workflow.)
What's important for color keying is that your color is bright, consistent, and bleeds as little as possible onto your subject.
Any good chroma keyer can pick up on any color value and tweak it's output so that it picks up a range of colors around that value. You could start with green, blue, red, muave, chartruse, aquamarine, or burnt sienna. Just make sure your subject doesn't share any colors with what you're keying.