Hi Scott,
I don't know about you but I have had four calls TODAY related to this topic. I was asked to "knock off" with the chemistry jargon so I won't go into anymore scientific lingo pertaining to plastics.
The plastics used by Chrysler were comprised of the cheapest materials they could purchase. They did not use a special "blend" to formulate a purposeful yellowing effect. Anyone familiar with neutral colored plastic knows that it turns yellow and brittle over time. Environmental conditions such as heat (engine compartment) will accelerate these changes. I have three NOS bottles that all exhibit varying degrees of yellowing. The photo below is an E Body example that displays various stages of discoloration throughout the SAME surface. I doubt those stages of "yellowing" were engineered into the color scheme of things. Get an NOS sample and notice the small air bubbles that are evident within the structure. Even the new "white" ones that are manufactured today have the same characteristics because they are basically the same plastic composite that was used 40 years ago. The "yellowish" aged version currently being reproduced has tan pigment added and the appearance is totally different than that of the captured air bubble look. The added color causes a denser looking type of plastic material. If you have both versions in your shop, check it out for yourself!
I also posted a picture of the carb decal I am making for you Scott. Would you like it to look faded and old like the original, or new like the one shown above it?
Just out of curiosity, do you restore/color your carburetors to look old and tarnished or new like they would have appeared 40 years ago?