my understanding is the original purple is based on the Plymouth and Dodge purple from 1970 used on Cudas and others. Here's the color chips on those cars, and you can see the purple is indeed metallic. Most people call it "plum crazy" but the dodge and plymouth names for this color are different (though it's the same color.) High Impact paint color "In Violet" or "Plum Poly."
http://ebay.com/itm/373777397617
http://www.ebay.com/itm/374232103091
But i'm not 100% sure this is the actual color. The rumor went like this, and it started with Ruttman and their Grasshopper chopper.... Ruttman was based in Dearborn MI, and one of the employees worked at the local Chrysler factory doing paint. When the 1970 models started being painted (summer 1969), the employee brought the extra purple paint from his paint gun home, and sprayed it on the Ruttman chopper. Management at Ruttman liked it so much, they made it the standard color for the Grasshopper. Now i know this is not Arco, but the other chopper makers (little indian for example), followed suit, and everyone was using this color for their chopper models.
i don't know if this is indeed true, but that's the rumor as it was told to me by the guy that bought all of ruttmans remaining inventory of parts in the 1980s.
i've been trying to find a good powder coat color that matches the original 1970 purple, without a lot of success. the colors i listed above are my best guess, though i have not personally tried them. here's one of my fails on that color, too dark....
http://www.pinrepair.com/minibikes/p/arco1970_easyrider_mine1.jpg
by the way, the easy rider arco i had above was awful to ride. it was a serious wheelie machine that wanted to kill you. i ended up selling it because it was just so difficult to not turn the thing over and it take off (without you) down the road.