There were a number of running changes in offered colors and usage. Know of many factory 506 Dark Olive Poly (metallic) GMCs. Go with the code and enjoy. Just remember current metallic size and clear coats can change how the final color will look on the truck. So work to a close match if originality is important to you.
72 Chevy chart
http://paintref.com/cgi-bin/colorcod...et&model=Chevy Truck&year=1972&con=koy&page=1&rows=50&size=large
506/WE5185 code usage
http://paintref.com/cgi-bin/colorcod...M&smodel=Chevy Truck&sname=Dark Green