Scott,

I know you are excited about your restoration. I have owned HG's OC3's and an OC4 dozer. The OC3 is yellow. Most OC3's were painted yellow with the exception of a very few of the early models when they switched  from the HG. The highway yellow found at implement stores and agricultural hardware stores works just fine especially if you add hardener. If you want to go first class you can go with PPG Ditzler 81985 or old Caterpillar highway yellow. I hope this helps and if you have other questions let me know.