Would someone knowledgable please explain to me the difference between the two?  And while you're at it...explain the difference between large pin and small pin and what makes one more desirable than the other.

Blue or green doesn't REALLY tell the whole story.

Blue was made after the green label.

Receiver markings vary:  my greens are marked AR-15A2, some blues will be marked "sporter" or other P.C. name.

In general:

Blue has the small pin, green large pin.

Blue has fence around mag release, green is slab side.

Later blues have sear block and no bayo lug. Greens don't have sear blocks and have the bayo lug.

Small pin is the size the military uses, so an upper can be easily changed, ala push pin.  

There are many exceptions when talking about Colt configurations. Colt was assembling rifles with whatever parts happened to be available at the time of assembly. There are many variations to the "norm".

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