Wouldn't i be great if reproduction plastic came...
 

If I were king and made reproduction plastic parts like seat backs, dash parts, arm rests etc...
I would do them in light grey or maybe even a darker grey. This way if someone needed to dye them, (This is usually the case! ) it would be much easier and take much less color to get them correct. Marks might also be less obvious.
Just sayin'!

Not a bad idea. I noticed that many plastic dealer parts are a yellowish tan under the applied color. Examples, interior of the underdash A/C duct on the '68, underside of plastic console on the '66, the wheel center caps on my '02 Tacoma where the silver has worn thin. All the same yellowish colored plastic.

I think it would be great if they came in all the colors and actually matched original.

It's normal for the cars I've seen it on. I can't say what's normal for all cars though. Like I said, I have seen the unpainted side of quite a number of products that had that yellowish tan color. On the other hand, I've seen plenty of plastic parts that were molded in color also. Volvo for instance has many interior parts that have different part numbers to match different colored interiors.

Reminds me of a 'Beach Boys' classic: "Wouldn't It Be Nice?"

I've always wondered about this. When you see a factory plastic part that is not completely covered in color ... was it painted or dyed after forming, or was it done while the part was formed by only using pigment on certain areas of the mold.

With injection molding I would assume all the base plastic has to be colored prior to forming, but with other types of molding they could be conserving pigment by only using it where it is visible.