I am trying to remember off the top of my head on the paint. One of my chapter member has a green OC-3 which he had me help refurbish. They were painted green as standard color for one year, I recall that as being 1952. Prior to and after the standard was yellow, but, you could order any color as an option. I have seen several original OC-3's of various years that were green, which means the dealer or purchaser preferred green and payed extra for this option. I tend to think this was probably a regional thing, some customers in areas may have not wanted to change.
I have seen different colors on these tractors, too. It was at one time fairly common for contractors to have all their equipment painted in their company color, one contractor in my area did and still does order all their equipment painted pink. Not just any pink, a special shade of pink. You can recognize it a mile away.
As to your timing. Remove #1 spark plug, put your thumb in the hole and turn the crank until you feel the compression pressure pushing against it. There is a hole on the front of the bellhousing on the right side of the tractor. These came with a plug screwed into the hole, it may or may not still be there. With a light you can see the flywheel thru this hole. On the flywheel is a timing mark, on a HG this mark is stamped SPK, I don't know about an OC-3, but it will be a noticable mark machined across the flywheel, not a scratch or chisel mark. Continue turning your engine over in the same direction slowly until you find this mark, then align it exactly in the center of the hole. This is where #1 cylinder should fire. Since I don't know which magneto you have I'll explain a way to set either. Take the cap off your mag, find #1 spark plug wire, align the collector ring, or what you may call the rotor button on a battery ignition(same thing, different names) before the #1 post, prior to it engaging the impulse. You will be able to turn the mag in the rotation of operation and feel a resistance to the impulse spring. Leave your mag in that position, which is just prior to firing #1. Bolt it back in your engine in this position with the top of the mag away from the engine, there should be only one way it will go in, be careful to keep the engine and mag in these positions. The flanges should mate correctly, and tighten properly. Then loosen both bolts a couple of turns, loose enough to rotate the top of the mag toward the engine block until the impulse clicks, this will be fairly loud, and you should be able to feel it thru the mag. If there is not much wear in the gears, and they are all in the correct position this should occur when the mag is approximately straight up and down or canted slightly in the direction of the engine block. Tighten the mounting bolts, and make sure the wires are in the correct firing order. This should be the correct timing, and your engine will run, but you may need to slightly adjust it some. Just make sure it will start using the crank, and has power, your timing will be as it should be. Once I did this, and someone had taken the governor drive gear out, then put it back 2 teeth off, since this gear also drives the mag, it drove me batty until I figured that one out. The mag was completely against the block, I had put the end of #1 spark plug wire in the timing hole, and could not get it to spark when the timing mark came up, which it should. Eventually after much hair loss I figured it out. On these old tractors I now remind myself that in all probability many people have worked on them, maybe some know what to do, maybe not. So, that can be something to deal with. Just keep this in the back of your mind and it might make things a little easier. You really should get a manual and parts book. There are two sources I know of for this, Floyd County Museum (641)228-1099, and Zimmerman Oliver Cletrac (717)738-2573.
Good Luck!!!
Andy