Temporary paint on a Knoll tulip table  

The original
non-shiny painted surface was great and the currently available new ones have a different type of paint; it's shiny and not quite the same.
The vintage paint/lacquer (whatever it was) had a slightly bumpy, non-shiny surface.
If I had a yellowing base where the paint surface is basically OK, I'd clean it completely and maybe even dab a bit of dupluted bleach on the painted surface to try to remove the yellowing (which is probably cigarette smoke, anyhow!)

Vintage tulip tables
...were coated in rilsan which generally has a satin finish. It does mellow with age and that softer patina and eggshell color is cherished by a lot of collectors. I don't know how they are finishing current production tulip tables, but to my eye they are far too glossy and bright, cool white, and they have a plastic look to me.
If your table base is in excellent condition (no chips or loss to the edge), you are very fortunate and I'd encourage you not to paint it at all. There are environmental factors that can cause it to yellow. Smoke, like Barry said, is a common culprit.
You might want to try cleaning it with some common household cleaners if you want it to not look as yellow. I've used Scrubbing Bubbles and Soft Scrub with good results. Soft Scrub usually has some bleach in it, and is mildly abrasive so I'd go easy on it, and try an inconspicuous area like under the top before doing the whole thing. It's also easier to control than dupluted bleach. Diluted bleach as well for that matter 😉
Oh, and all of this assumes that the base was supposed to be white in the first place. Knoll actually made Saarinen pieces with a very soft, light gray colored rilsan and also an ecru or tan color ( as well as black, of course) by special order.

Pegboard
has excellent ideas.....I take my back my diluted bleach suggestion in favor of Soft Scrub.
Pegboard's also right about the current Tulip's having a tacky, shiny and thinner surface that chips easier than the older finishes.
I bought a single vintage Saarinen stool on eBay when I first started assembling furniture for the house I've lived in for almost 4 years and i did have the fabric redone (as it was nasty torn vinyl). My upholstery friend had a remnant of genuine Knoll wool red fabric and did a swell job on the upholstery and we both decided to leave the white surface alone. it was a good decision.

Of course
If you like a new looking surface, that is great. Sell yours to one of us vintage fans and buy a new or used newer one. That will save you the trouble and time spent possibly lowering the value of your table. Also, I get what you are saying about the water soluable paint. It is unlikely to give a surface that has a quality look. I am thinking of children,s washable paint and how that appears chalky and I think it also flows poorly. And after you clean the base and it is not just smoke, you just may have one of the non white ones. I have seen the tan and have the gray. They both could be mistaken for a darkened white if you never knew there were actually these colors available. Post a pic if you can.

Thanks everyone!
I actually went ahead with a slightly diluted bleach solution and it took the yellowing off immediately. The soft ivory of the rilsan is not nearly as harsh a contrast to the formica top now. Thanks so much!
I would post photos but my flickr account is not accessible right now.