Red (#FF0000) is a medium, highly saturated red hue. Pure sRGB red is the loudest colour on a screen and the classic signal for errors, sales and stop signs.
In RGB it is made of 255 red, 0 green and 0 blue. Its HSL hue angle is 0° with 100% saturation and 50% lightness, while its perceptual OKLCH lightness is 63% with a chroma of 0.258. For print, a starting CMYK mix is 0% cyan, 100% magenta, 100% yellow and 0% black.
As a text color on white, Red has a contrast ratio of 4:1, which passes AA only for large text and UI parts. On black it reaches 5.25:1 and passes AA for normal text. If you use Red as a background, put black (or very dark) text on it. The APCA lightness contrast is Lc 40 for black text and Lc -70 for white text.
Its perceptual complement is #009AAC, close to dental teal. Colors that look most similar are scarlet, crimson, raspberry. People with red-green color blindness see red quite differently (ΔE 25), so do not rely on it alone to signal meaning.