Traditional European heraldry (coats of arms) has a lot of rules regarding how designs can be constructed. One of the fundamental rules is this: no metal on metal or color on color. There are two metals: gold (represented as yellow) and silver (represented as white). All the other hues that can be used are “colors.” The rule is that colors can’t be placed next to other colors, only metals. Metals can’t be placed next to other metals, only colors.
This may seem like an antiquated piece of trivia, useful only to those who are desperate to be the fifth cousin twice removed of the Duke of Cornwall or somesuch, until you look at street signs. In the U.S. (and those parts of Europe and Canada I’ve visited) almost all street signs follow the heraldic metal on color and color on metal rule. A U.S. stop sign is a metal (white/argent) on a color (red/gules). Highway direction signs also (white/argent on green/vert). Speed limit signs? Black/sable on white/argent. Those few signs that don’t follow the heraldic convention, such as construction signs with black text on an orange field, are much less noticeable than the vast majority that do.
What’s going on here? Heraldry was originally designed so that painted shields and banners would be clearly visible at a great distance on the battlefield. For that to happen, you need to have a lot of contrast. As it turns out, with the pigments they had available, white and yellow had the best contrast against the other pigments. Thus the avoidance of color contrasted against color or metal contrasted against metal. Even with modern pigments and special reflective surface treatments, the rule pretty much holds up, so sign designers follow it even if they don’t know where it came from. Next time you’re on the road, try to find signs that break this rule. If you do find one, note that the contrast is poor compared to signs that follow the rule. Same with signs on buildings.
This rule can be useful when you are designing a color scheme and want to highlight a focal area of a painting. If you follow this old heraldic rule, you will have all the contrast you could need.