It wasn't yellow. It was white and turned yellow with age like real Ivory does.
Some years ago(early 80's, IIRC) a bunch of knifemakers started getting on Westinhouse about this and wanting it to never turn yellow but stay white forever, and Westinghouse knowling that knifemakers make less than a pimple on their backside when it comes to buying power and were only producing the stuff for them to begin with, just discontinued it, completely!!
The only way you can find it now, is if some other knifemaker or such has some old stock in their shop and will sell you some.
Edited to add: Welcome to Bladeforums! Enjoy!