Red passenger cars in the 30s

This was referenced in another thread but it's made me curious. Is there any clue as to why the painting of some of the passenger cars red? It appears that a few were painted in the early '30s and others as late as '39. Two baggage/coaches early followed by 215. Then two mail/coaches that also had their partitions removed and the windows boarded up. Then a few coaches with the last refurbished and painted in '39. Most all of these cars were sold to Mexico in '42.

So, why? Was this a way to store extra cars and stave off deterioration? Or were they used to move workers for WPA work in the area ( I read somewhere that there were work trains put together for this reason around the country)? Were they supplemental to various outfit gangs?

Just wondering if the reason is known?

Phil