Yellow Paint?

I just got an early D 17 and a 54 WD 45 and they both have some yellow paint. The 17 has the engine block yellow and rest AC orange. The 45 has the torque tube yellow and the rest AC orange. Looks like factory primer and not owner paint. No construction background parts that I know of unless parts swap sometime. Where did the orange go or where did the yellow come from?

I have a B (with a belly mower and plow) that appeared to be yellow. When I was prepping it for paint, I found some orange in spots, but also found yellow in places that made me think it might be original.

Remember seeing factory replacement blocks and other big components in uncle's dealership warehouse in the late 60's and early 70's, all in factory yellow primer.

Had an interesting thought- given the transition years of the WD45 and D17 and the comment of replacement parts being in dealer stock without orange paint, did they possibly leave parts unpainted in case the farmer had a tractor repainted in PO2 vs. PO1? I know there were many WD/WD45s repainted with PO2 and even cream wheels when the newer D-series came out, as farmers and dealers would "update" the look of older tractors.