My LGS has a vintage Colt SP1 rifle (1966-67) which appears to all original. The ejection port door spring is broken and needs to be replaced. The original spring has that old patina look to it. Is there
a way to age a newer ejection door spring to match the darker color of the ejection port door?

It is easier to just get an old spring.  If you purchase that SP1, just IM me and I will send you an original aged spring.

He does have a great question though.
I have been pondering a way to color the later, silver colored springs to look more appropriate on earlier builds.
Of course, the ultimate rush is to find, and use an original carbon steel spring.

It appears to me the stainless? 70's Colt springs are ferrous as I've seen more than a few that had corroded themselves to the rod and were rusted beyond recovery and crumbled once removed.  Lots of PB Blaster to get loose.  Probably a night or two in a weak solution of Miracle Grow should do the trick.  Newer springs may have a low or no ferrous metal content in mix and might not be as sensitive as the 70's Colt springs to a little patina.  Coke Cola probably do the same thing.