Thank you. That is exactly what I was looking for. Found two cans on Ebay.
Bob in Phoenix
If you want to be precise, according to "Technical Manual TM9-1340-202-12- Operator and Organization Maintenance Manual for the 762mm Rockets MGR-1A and MGR-1B Honest John Rocket System", dated October 1969, the US Army specified that rockets in the field were to be painted "Olive Drab Enamel Color No. X34087." A Google search reveals several brands of paint that match that specification.
Scale military modelers have heated debates about the exact shade of olive drab, based on extensive research and documentation. In model rocketry circles, even in scale competitions, I have never seen that level of scholarship applied to the topic. At most meets, anything that is remotely olive drab green is accepted as being true to the prototype.
In reality, it is hard to find a missile that has had more paint schemes that the Honest John. Test rounds, field deployments including in many allied and NATO armies, static displays, and various paint schemes on display models for Estes, Centuri and others provide an almost unlimited variety of paint schemes that could be considered "canon" for those who become obsessive over such things.
Best of luck with the build. That's a great model and a challenging build.