I did some research some years ago on the '64 Pontiac color Sunfire Red, learned a good bit.
"Poly" in the marketing name for paint colors was short for "Polychromatic". As Bruce mentions, this is the effect created by metal flake added to paint that changes the "look" or color of the paint depending on the angle that light hits the randomly embedded flakes.
I'm not positive, but I believe Dupont came up with the term polychromatic to describe the paint that Dupont developed in the '50's by the addition of Aluminum flakes and GM paint colors containing Aluminum flakes began to include "Poly" in their marketing name.
Cadillac marketed their Fire Frost paint colors for a time, a premium price paint option. These colors as I recall used a larger flake to create a more dramatic Polychromatic effect.
In '64 anyway, Dupont, PPG, and Rinshed-Mason supplied paint to GM. Each Plant used a specific supplier. The "home" Pontiac, Mich Plant used Dupont as their supplier. I do not know what supplier was used at Norwood.
The subject of '73 Brewster Green has been discussed in this Forum as to whether it was a "Poly" color or not. The color formula for it did not include Aluminum flakes. Some have claimed that original cars in Brewster Green exhibit a "metallic" appearance. My own '73 TA was Brewster Green though by '78 when I bought it for the first time, it had been repainted Black. There is a bit of Brewster Green still in hidden areas but I have not tried to look for the "metallic" appearance.
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