Valentine’s Day is nearly upon us, that sweet Hallmark holiday when you can have anything your heart desires, so long as it’s red. Red roses, red nighties, red shoes and red socks. Red Oreo filling, red bagels, red lox.

As it happens, red is an exquisite ambassador for love, and in more ways than people may realize. Not only is red the color of the blood that flushes the face and swells the pelvis and that one swears one would spill to save the beloved’s prized hide. It is also a fine metaphoric mate for the complexity and contrariness of love. In red we see shades of life, death, fury, shame, courage, anguish, pride and the occasional overuse of exfoliants designed to combat signs of aging. Red is bright and bold and has a big lipsticked mouth, through which it happily speaks out of all sides at once. Yoo-hoo! yodels red, come close, have a look. Stop right there, red amends, one false move and you’re dead.

Such visual semiotics are not limited to the human race. Red is the premier signaling color in the natural world, variously showcasing a fruitful bounty, warning of a fatal poison or boasting of a sturdy constitution and the genes to match. Red, in other words, is the poster child for the poster, for colors that have something important to say. “Our visual system was shaped by colors already in use among many plants and animals, and red in particular stands out against the green backdrop of nature,” said Dr. Nicholas Humphrey, a philosopher at the London School of Economics and the author of “Seeing Red: A Study in Consciousness.” “If you want to make a point, you make it in red.”

What is it, then, to see red, to see any palette at all? Of our famed rods and cones, the two classes of light-sensing cells with which the retina at the back of each eye is supplied, the rods do the basics of vision, of light versus shadow, tracking every passing photon and allowing us to see by even a star’s feeble flicker, though only in gunmetal shades of black, white and grim. It is up to our cone cells to capture color, and they don’t kick in until the dawn’s earylish light or its Edisonian equivalent, which is why we have almost no color vision at night.