El rojo es el color que se percibe ante la fotorrecepción de una luz cuya longitud de onda dominante mide entre 618 y 780 nm. Se asemeja a la coloración de la sangre arterial humana. Se encuentra estandarizado en catálogos de colores e inventarios cromáticos, sirviendo este rojo «estándar» como modelo y referente para el color rojo. El rojo estándar puede verse en el recuadro de la derecha, arriba; los valores que se dan debajo corresponden al mismo. La denominación de color «rojo» abarca al conjunto de coloraciones similares al estándar, rojizas.

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Red is the color at the longer-wavelengths end of the spectrum of visible light next to orange, at the opposite end from violet. Red color has a predominant light wavelength of roughly 620–740 nanometers.Light with a longer wavelength than red but shorter than terahertz radiation and microwave is called infrared. Red is one of the additive primary colors of visible light, along with green and blue, which in Red Green Blue (RGB) color systems are combined to create all the colors on a computer monitor or television screen. Red is also one of the subtractive primary colors, along with yellow and blue, of the RYB color space and traditional color wheel used by painters and artists. Reds can vary in shade from very light pink to very dark maroon or burgundy; and in hue from the bright orange-red scarlet or vermilion to the bluish-red crimson. Red is the complementary color of cyan. In nature, the red color of blood comes from hemoglobin, the iron-containing protein found in the red blood cells of all vertebrates. The red color of the Grand Canyon and other geological features is caused by hematite or red ochre, both forms of iron oxide. It also causes the red color of the planet Mars. The red sky at sunset and sunrise is caused by an optical effect known as Rayleigh scattering, which, when the sun is low or below the horizon, increases the red-wavelength light that reaches the eye. The color of autumn leaves is caused by pigments called anthocyanins, which are produced towards the end of summer, when the green chlorophyll is no longer produced. One to two percent of the human population has red hair; the color is produced by high levels of the reddish pigment pheomelanin (which also accounts for the red color of the lips) and relatively low levels of the dark pigment eumelanin. Since red is the color of blood, it has historically been associated with sacrifice, danger and courage. Modern surveys in the United States and Europe show red is also the color most commonly associated with heat, activity, passion, sexuality, anger, love and joy. In China, India and many other Asian countries it is the color of symbolizing happiness and good fortune.

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