Ken:
If I'm not mistaken, Kohler has a custom-color program. I think if you're willing to pay for it, they're willing to crank it out for you. Worth an email.
Today's porcelain on cast iron is better than yesterday's. It's now a premium product, not a mass-market one produced to a price point. There are fewer impurities in the cast iron, fewer in the porcelain, and today's application methods put down the layer of glass frit more evenly than before.
My understanding is that the Delafield we had installed in '59 was a $59.95 sink.
P.S.: The "Turquoise" color you're looking for was Kohler's No. 08, Cerulean Blue. Their "Yellow" was No. 18, Sunrise.