The NG and Short Line Gazette had a three part article on NG tank cars beginning  Jul/Aug and Sept/Oct 1978. The third part Nov-Dec 1980 was on Conoco tank cars.
The paint on Conoco cars was silver with black reporting marks and d data and dark green large Conoco. By late 1930s all painted black with white lettering. No mention of red cars.
I hope this is of interest and adds information.
Chuck Seemann