Adjoining states went away with FOPA 1986 at the federal level, as of 5/19/1986 - the same piece of legislation that allowed interstate/mail order ammo sales again, and outlawed new machineguns.
Colorado does still have a law on the books specifically ALLOWING purchases of long guns from FFLs by persons from adjoining states, but has no law forbidding purchases from persons not in adjoining states anymore, so those are legal, too.
California law requires purchases by its residents to be executed with a dealer record of sale (DROS), which can only be done by a California FFL - so while Colorado law would allow a California resident to purchase a long gun from an FFL here, California law forbids that transaction, and ATF/NICS will not approve those transfers.