Interesting Daryl,
If your trailer had a 'proper' bridge plate, (one of the pressed ones like the land rovers had) it would have been yellow originally. So, it must have been stripped down to the metal at some stage.
My trailer has a locally made plate, which is a simple disk of steel, without the pressed rim, and it had several coats of yellow under multiple coats of olive drab. From all of the stone chips etc, I'm pretty sure that it is the original one.
A bit of driving around on dirt roads should see your newly painted plate pick up a fair bit of weathering fairly quickly!