I believe a lot of it has to do with resale value. People want to be able to trade in their car for maximum value, and picking the color with the widest appeal helps with that. This seems to have created a feedback loop where car manufacturers ship more of the "boring" colors because that's what customers are buying.
This also applies to things like computers: A cursory search on ebay shows that a silver iMac generally sells for more than an equivalent yellow iMac, for example. For someone who trades in their old equipment when buying new, this is a big deal.
Odd colors actually have the best resale value on the used market. A yellow Porsche is very hard to sell new. Very few are made and sold in yellow because dealers don't spec them that way. 10 years later, if you try to sell a black Porsche, you have 72 others competing with you. If you try to sell a yellow Porsche, the buyer will fly from across the US to buy it from you.
>Very few are made and sold in yellow because dealers don't spec them that way
Dealers aren't speccing them yellow because they don't sell as well as other colors. If people were lining up for yellow Porsches, dealers would absolutely be ordering more yellow Porsches.
Odd colors may be priced differently at the top of the market for used cars (I don't know), but that's just not true for the typical car used buyer.
The problem isn't that fewer unusual colors are made.
The problem is that no unusual colors are made in almost all car models.
There are a very small handful of models that have a few particular colors beyond the basic black/white/red/silver/dark blue options, but otherwise manufacturers have decided that because fewer people want yellow (or bright blue, or pink, or orange, or magenta), no one gets those colors. Ever.
Even the new Miata's color options are gray-scale, red, very dark blue, and sand. Worth mentioning that you can just buy wraps for wacky colors while protecting your paint and resale value, maybe that's part of it.
I specifically wrote that a new yellow Porsche is difficult to sell.
People will travel for used cars. New car buyers want to pick a car. A few new car buyers do spec out custom orders and wait, but it's very rare. In general, that's atleast how half the crazy colors come into the market.
This also applies to things like computers: A cursory search on ebay shows that a silver iMac generally sells for more than an equivalent yellow iMac, for example. For someone who trades in their old equipment when buying new, this is a big deal.