Hi, Dave,
I would tend to aggree in general with William's comments, because I believe silver was used for a long time.
Jerry's data from 1926 supports the notion that black would be OK into the 30's.
We received caboose #04990 in Boulder in the late 70's. When we checked it out to answer the quiestion of railing color, they appeared to have been painted white. That caboose was retired in the early 50's.
Take it for what it's worth and do whatever you feel most comfortable with. There can be evidence found to support anything or everything.
I have a friend who built an On3 caboose of the period Jerry's data supports, and it turned out to be a beautiful model.
I only saw them with silver grabirons in my time, but I don't go back that far (30's!).