Q. What is the rule on hyphenating multiple colors? For example, it would be “a black-and-white photo,” but you never see any other color combinations hyphenated (like “a pink-and-yellow scarf”).
A. Putting something in black and white has been a thing for centuries; according to Merriam-Webster, the noun phrase black and white to mean writing or print dates to 1569. Once color photography became popular, the adjective black-and-white—with hyphens—was probably inevitable.
A pink-and-yellow scarf, on the other hand, or one that’s red and white or blue and green or any other combination you can think of? Those are arbitrary combinations. But you should hyphenate any one of them as a preceding modifier regardless (as at the start of this paragraph), at least if you’re following Chicago style. Hyphens will make it more obvious that you aren’t referring, for example, to a pink and a yellow scarf.
This issue is not, however, black and white. For more on this subject (including why we’ve left the hyphens out of black and white in the first sentence of this paragraph), see “Compound Modifiers After a Noun: A Postpositive Dilemma,” at CMOS Shop Talk.