I would refer you to the esteemed Honorable Hedley Millar's essay "The White Lead Question" in 1912's preeminent journal for purveyors of pigmentation in His Majesty's Empire, Paint, Oil and Drug Review, in which he provides an extraordinarily thorough overview of the then-pressing question for the maritime industries of the world that should satisfy even the most curious connoisseur of palettes.

But the 2020 translation of what he's bloviating about is a bit simpler: it came down to cost and effectiveness for corrosion above the deck (white lead/zinc - was not just resistant but also let crew see rust easier, plus vastly improved visibility for navigation if the superstructure is painted that way) and killing off marine life below deck but above the water line (black lead) and underneath the water line (red lead/copper) where it was a constant battle of trying to poison barnacles et al so they'd die off but not corrode your paint job. Different colors were likely because white>red>black for cost (you saved the really expensive stuff for underwater where the wear was worst) unless you went really cheap on the white above deck, which was cheaper up front but then created more maintenance.

Oddly enough even that essay notes the downside, which is that lead poisoning was known back then too (although he seems to be a lot more concerned about the cost of paying sick workers than them getting sick!), and paints have gotten better and less toxic since then so color schemes have been freed up quite a bit.

Can't give you a good answer on the smokestacks, so hopefully someone else can chime in for them if they know a source. An educated guess would be that the alternating colors increase visibility in certain circumstances like ice or fog; white on white can be really hard to see in the sky or against a background. This is one reason why the modern day US Coast Guard paints its cutters white (which stands out against sea level green or blue), but most of its helicopters and icebreakers are orange instead. Unlike the Navy, they generally want their ships to be as visible as possible for search and rescue purposes.

Edit: Heh, just noticed the ad at the end of the article soliciting bids for 170000 pounds (85 tons!) of white lead paint for the US Navy's Norfolk yard, which at the time was home port for the Atlantic Fleet. Hadn't realized the 1912 paint scheme was still Great White Fleet rather than Mare Island Grey, but that should give you an idea of the quantity required if you were going to do multiple ships in a single color.