In reviewing the various photos of 18 from the 30s to her end of service, she had a bright silver smoke box until the late 40s when her tender lettering was changed and they switched to a graphite color on the sides and stack and possibly a duller silver on the front, that is a toss up due to weathering and grime etc.
From photos it almost looks as though in the 30s she did have Aluminum painted lettering because it doesn't show up as prominently as it does in the early 40s. Sadly I haven't seen any good pics yet dated between 1942 and 1948.
My friend and I are pondering if after the line was severed in 38 and the locos no longer went from Mina to Sparks for service, if they weren't just paitned and stenciled in Keeler and the official Aluminum and later SP Grey were not available so white was used. This is just an idea we have debated.