In the few photos that I have seen, the markings appear to be white and match the white of the national insignia fairly close. I would think that yellow would show more of a difference in tone/shade.
I do not know why some aircraft have their cowlings marked and others don't. Could be a date/time issue as markings progressed or perhaps different sections within the squadron?
http://collections.naval.aviation.mu...eb%2FQuery.php
https://www.worldwarphotos.info/gall...march-21-1945/
https://www.asisbiz.com/il2/Hellcat/...r-1945-01.html