I’m fiddling with realistic shaders only for some weeks now, but the first question is to ask: how close will your camera be to the keyboard. If it’s something in the background, you’re fine with you’re shader. If one key fills your picture, it will need some tweaking.
I had the background problem with the piano of some DAZ-model: I simply set the color shader to RGB 245-242-232, hightlight to value 30, shininess to 7. That’s enough for something solely living in the background,
On something for the foreground I would start with the same simple background shader, then add some reflection value under 10. Adjust Highlight and Shininess accordind to your surrounding lights. Maybe use some procedural shader in the color channel, because ivory is mostly uniform, but not at all if you take a look closer.
Ah, and subsurface scattering will be essential for realism. It’s not like a pearl, but even a key on a piano has some slight, really slight SSS. How much, will depend, as usually, on the surrounding light.
Sorry; I can not give some simple solution, because every shadesr values depend on the lights, which will hit them. (I’ve learned that the hard way the last weeks )
edit: And what Garstor said.