Red and black
Not so the red/black opposition. To be sure, hues may be measured in angstrom units, arraying them in a vast rainbow, but the only contrast that seems objectively valid is between white (total fusion of colors) and its opposite black (the absence of color). And we do speak of black-and-white contrasts. Among other things, piano keyboards and old movies reinforce the perception of this complementarity. However, red is just a hue among countless others, with no objective or necessary companion, except as convention dictates. In traffic lights, for example, red contrasts with green.
Why then the red/black antinomy? The answer goes back to the writing conventions of the ancient Egyptians, whose beautiful hieroglyphs we admire in our museums and libraries. In Egyptian papyri black is preferred for the main body of running text. Red, which occurs less often, is reserved for headings and words that need to be emphasized, italicized as it were.
Among the Egyptians this contrast was in part practical, reflecting the ready availability of black ink (carbon black) and red (hematite, or red iron oxide). There were also symbolic overtones. The Egyptians often called their own country Kemet, the Black Land, acknowledging the rich dark soil brought from East Africa by the annual inundation’s. By contrast red was associated with the desert, a potentially dangerous, but inescapable accompaniment of the Black Land.
These symbolic associations faded, but not the idea that black is to be preferred for main texts, red for exceptional indications. Medieval scribes called the use of red "rubrication." Even today, we use the term "red-letter days," which originally referred to special feasts and saints' days in the calendar. At the beginning of her career, the Theosophist Annie Besant wrote a book on "black-letter saints," worthy figures but not as famous as the red-letter ones.
Today a version of the contrast occurs in bookkeeping, where accountants traditionally enter debits in red. This practice has influenced ordinary language. No one wants to be "in the red," while the assurance that one is "in the black" is calming.
The bookkeeping practice draws on the association of red with danger, which may be biological in origin. In the case of the roulette wheel, however, this association is not present. Given the mathematical nature of the odds, a better would be foolish to avoid red.
In Stendhal’s great novel, Le Rouge et le noir (1831), red refers to a political career, possibly a revolutionary one, while black means choice of the Church. Roughly they correspond to to our Left and Right. The contrast could also occur within factions of the Left. In 1834, if memory serves, two flags were unfurled in Paris: a red one for socialism, a black one for anarchism. Red has continued to be favored in the symbology of Marxist regimes, while some Anarchists unfurl the black flag.
It is a curious fact that a custom that started in ancient Egypt lingers in two realms where the red/black contrast is significant: in gambling and accounting. Perhaps someone should alert the Las Vegas hotel, the Luxor (where I stayed last year). Both types of endeavor, gaming and accounting, figure in the business side of the hotel.