It's not completely straightforward. The thing is, the colour values are defined in a file named colorscales.csv. The definition of which country uses which colour is in country.csv. The problem to migrate the CORE colour scheme to vanilla is CORE using a different country.csv than vanilla. I'm not completely sure if the vanilla tag entries are the same as CORE's and just the added countries in CORE are different. If so, there should be no problem in copying the CORE country.csv over the vanilla one. If the entries are different for more countries than the added ones, you might get some odd colour combinations. The best way to find out would of course be to copy those two files into vanilla. If you keep backups of the vanilla files, you can just roll back the changes if things go wrong.
As for the other graphic changes, just copying the CORE gfx dir (except tech) over the vanilla dir would work.
