Freight car colors - black or boxcar red?

[nachoman]

Modern freight cars come in a variety of colors -

But between 1900~1930 it seemed most cars were either boxcar red or black. Well, at least I *think* they were - afterall the photographs are B&W.

Was there a rule to what colors cars were painted? It seems like boxcars were primarily red, while other cars were black. Would it be entirely unusual to have a black wood boxcar during this era?