how many colors  

Impossible
Impossible

wow
Thanks for posting the color chart!

That forceful
Impossible, sounds very unlikely.
First of all, over the years I have seen how colorants changes formula almost overnight. Colours that were achieved with one set of colorants changed into other sources or synthetic colorants. Especially in polyester this is quite likely because polyester colorants are more critical. Many metal based colorants that can be used in thermoplastics destroy the polyester's ability to stratify, so they can not be used, others like cobalt act as a catalyst and unless you want to use them as catalysts, you can not use them either.
A large part of discolouration is caused by ultra violet light, so a chair that is continiously outside would show discoloration whereas an inside chair of the same age would not show it so, impossible? certainly not

I meant
exactly what Koen said. I just didn't say it nearly as fancy 😉

Well put Koen. I deal with...
Well put Koen. I deal with batch products every day. If a product has a problem, it can be
tracked with the batch #. (a trusted supplier may change their colorant supplier!)
And SDR is correct. A navy shell is somehow welded to a chain w/dog at a gas station in
Queens, NY. I pass it nearly every day. It is a very different (faded) color on the seat, having spent
years outside.
So many of the early shells were ordered by institutions in batches of 1000 or more.
I would guess it would be very easy to adjust the color to be custom? That may account
for variations in color? And also UV light will cause fading.
The color chart i posted is fascinating, i agree.
But i imagine many color changes happened since '62?
And yes, certain dyes change over time, and mixed with others....
Not necessarily exposed to light.

.
Some are factory coated the color is not integral to the shell body...The chart is a very interesting find...

First off
First off that chart represents the chairs manufactured in only one year - not the entire consecutive run of chairs. For example I own a blueberry blue armshell that isnt listed on the color chart. I've also seen some of the watermelony red colors of Olive's description and a bright canary yellow that is also absent.
I'm sure many colors were added, and probably many were removed. It has always been my understanding that there is a great difference between what is considered "rare" and what is not.
According to Roan John Barrion of Modern Love.com (who once quested to have a shell chair in every color): Rare indicates a scarcity of quantity - usually reserved for 'contract' colors. Contract colors needed to be special ordered, whereas a standard color could be bought in any amount from a dealer.
Rarity should not be confused with desirability. Red-Orange is a standard color - but because of its brightness is very desirable. Sea-Foam Green Light, however, is technically more rare. (I paraphrased this - but I have posted his exact quote elsewhere on the forum).
According to Mr. Barrion the rarest of colors is commonly considered hot pink. This may be true in terms of production line but Mr. Barrion reports of his unicorn - a grape purple (I shudder to think) shell that he has only seen in pictures.
Its also not impossible to think that Herman Miller would accept proposals of specialized custom colors for a large enough quantity.
The book "Modern/Classic" talks a lot about the variations and permutations of the plastic/fibreglas chairs - but doesn't go far enough. What is needed is a comprehensive compilation of Herman Miller sales materials & anecdotes that chronicles exactly when new colors were released, when changes to the frame and bases were made, and estimates of which quantities were produced.
Anyone near Michigan wanna get on that? ;-D

Well put. About what i was...
Well put. About what i was trying to say and thinking...
(not much of a writer here.)
Most seem to know of differences over the years, but a few examples on e-bay
right now add a bit to the learning curve.
One is near mint, rope edge, early Zenith.
The other blue one is a mess in my opinion.
And the going price reflects that...
http://cgi.ebay.com/EAMES-HERMAN-MILLER-ZENITH-SHELL-RAR-ROCKER-CHAIR-MI...

Here is the blue one...
Here is the blue one...
http://cgi.ebay.com/Eames-Herman-Miller-Arm-shell-Chair-Danish-Modern_W0...

rockland,
I've seen the blue one too. Any idear what's the cause for the bright spots on it?
So "Indiana Jones and the grape purple shell" is due. I'd watch that one! (And i'd also like to see that photo, too)

I know the purple armshell he speaks of......
One ended up in Japan mounted on a later rocker base, here are the pics and the link.
Behold, probably the rarest color, purple:
http://www.deco-boco.com/catalog/eames1/eames1_128.html

purple, purple,
purple, purple.
Though cracked, and on a repro rocker base, I would love to have it.....
http://www.deco-boco.com/catalog/eames1/eames1_128.html

BTW,
I'm hotlinking these photos, so download them. If the site ever goes down, you may never see this one again.

wow.
TFS!

i have seen a million ...
i have seen a million of the chairs but never the purple and it really is cool
I wonder what mental institution they are still at.