I've got this EA cast lower with gray finish.  Is it parked or in natural color?  What can I do to make it match modern black uppers?

Refinish or paint it. It is anodized and dyed that color most likely. I have never seen an upper that would match it. Even the old Colt and old Eagle Arms are a different shade of grey.

Nothing is exact unless it comes from the same lot.  Refinish both upper & lower, and you will be all set.  It is easy and doesn't cost much at all.  Check out this link for more info on a simple "how to" guide.

www.glocksunlocked.com/forums/index.php?showtopic=10200

Here is a upper/ lower combo I did in dark grey.....

Many years ago there were gray uppers that matched the EA lowers.  The EA lowers are very close (almost exactly) to the dry moly gray finish of USGI mags (Sandstrom dry moly).

I will be stripping off some crappy looking Krylon from an EA lower next week, and spraying with Norrell's flat black.

BTW, you can parkerize steel, but not aluminum.  

I like that gray-black on Stickman's picture above!