From "Comparative Naval Architecture of Passenger Ships," Philip Sims (M), Naval Sea Systems Command:
"The use of side coaling ports explain why the almost universal hull color of 1900 vintage ships was black. Since the sides would be inevitably streaked with coal dust, black was the best color. Photos of the WWI hospital ships, such as the Britannic, show smudges along their white sides. Modern white hulls are possible only with oil fuel. The water tightness of side coaling ports after years of service would be questionable. Coaling chutes would continually bang the port lid. The red lead joints would have lumps of coal crushed into the soft material every time that it was closed. The crew had to reach into the dark and dirty trunk through a
small hole to attach hooks and screw them tight and had no means of inspecting or testing the tightness of the closure.
The Titanic has been widely criticized for having
low (although closely spaced) transverse bulkheads
forward. Note that the loading doors are just below “E Deck” at the top of “F Deck”. The Titanic’s formula of low-height but closely spaced
bulkheads was in order to protect the coal ports. In a bit of irony, probably because she was such a new ship, the Titanic’s coal ports appeared to have been essentially watertight during the ships actual sinking since there was not a rapid increase in flooding rate when the waterline reached them."