Color Matching Kohler
Kohler can custom match your color for you, it will cost a bit more but it is an option. I will talk to my rep in the morning and post the information tomorrow.
Not sure this thought ever got completed, but I have a situation similar to the one above, need to custom match to an older Kohler. Two possible solutions, one "official", one not-so:
Official
Conversations with a Kohler Platinum dealer, Kohler, apparently they will custom order back to most custom/older colors, but there is some lead time, and they won't do it on every toilet, just some. Best to work out the details through a good dealer. Mostly this will be the best option for you, assuming the match is reasonable on a variant you'd like.
Unofficial
Ironically, the colors are not all that varied, and some manufacturers are pretty close to the others, so if the color is not too exotic, you might be able to match it. One trick, plumbing supply suggested, is to go get a color match on the toilet seat, one company makes most of them, they have tables from Kohler and others to their colors, and then do the reverse to another brands of toilet. The grey we were looking at turned out to match up with an Eljer, current product (or perhaps I should say, semi-current, given the Eljer off-shoring/situation/mess) pretty closely. YMMV.
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One irony, talking again with the Kohler dealer, they have tons of fashion/designer colors, but they sell mostly white and off-white.