IMHO, the 968 will always be a toy type of car for most in the future.
I would think only a small percentage of cars with low miles, full service history, rare colors
and rare options will ever really be "collectable". Jeffs cab being a great example.
That being said, I also have the opinion that just because some colors are rare, does not make them valuable.
They are rare colors because people did not like them when the car was in production and did not order them.
Using Corvettes as an example, when watching big car auctions, colors do not really effect pricing. Cars with desirable options such as fuel injection or the 435hp cars bring the money. In some cases I think the non desirable colors bring a little less.
The newest market craze is survivors. Cars that have stood the test of time with only maintaince. No repaint on the body or engine parts.