Answer: Purple

If you survey the colors found on all the flags of nations around the world, you’ll quickly find trends emerge. Shades of blue, green, and red dominate the color palettes of national flags (red is the color most used as a primary or accent color) with variations of each, as well as yellow, white, and black appearing frequently as well.

One color is most notably absent from all but two national flags: purple. There’s no superstitious or mysterious reason for the lack of purple in national flags, however, as the reason to abstain from the use of purple was a completely pragmatic one. Historically, purple dye was either completely inaccessible to people around the world or, when it was accessible (mostly to people around the Mediterranean region), it was obscenely expensive.

Before the invention of synthetic purple dye in the mid-19th century, the only source of truly purple dye was sea snails in the Tyrian region of the Mediterranean sea. The process for extracting the dye (either by milking the snail’s body or simply crushing it up) was very labor-intensive and it could take upwards of 12,000 snails just to create enough dye (1.4 grams) to color the trim for a single garment. It was so costly that it fetched its weight in silver at Colophon, in Asia Minor.

As such, it was simply impractical to use purple for anything short of royal garments. Even emperors and kings sometimes cringed at the price—third-century Roman emperor Aurelian famously forbade his wife from buying a Tyrian purple silk shawl because it literally cost its weight in gold.

We did mention above, however, that you can now find purple on two national flags. The flags of Dominica and Nicaragua, both adopted in the 20th century after the advent of synthetic purple dye, have small amounts of purple in them.