2021 update: Back in 2016, Aker offered a number of tubs and shower bases in retro colors. Alas, checking for this update I see that their color palette is now narrower; I called the company and they confirm the colors available are the ones shown on each page — there are no special order colors above and beyond these.
That said, I see you can still get tubs (and alcove shower products, check their site) in two shades of gray, two off-whites, and black (and white, of course.). These are not always easy to find in tubs made today. Read the Aker page for yourself to learn what materials are used to make these products.
Where to find these: Aker — a manufacturer of bathtubs, shower walls, and more
Thanks to Lynne and Bob for leading me to this manufacturer!