One colour has to be written several different ways depending on who is asking: a hex code for CSS, RGB for a design tool, HSL when you want to nudge the lightness without changing the hue, CMYK for a printer. This converter takes the colour in any of those notations and shows all of them at once. It also answers the two questions that usually follow — whether white or black text will be readable on it, scored against the WCAG contrast thresholds, and what the lighter and darker versions look like when you need a hover state or a full scale for a design system.
Hex for fixed brand colours, because it is compact and universally understood. HSL when you are generating variations, since changing one number gives you a lighter or more muted version of the same hue instead of guessing at three channels.
Treat it as an approximation. Real print colour depends on the paper, the ink and the press profile, so a printer will convert using a colour profile rather than a formula. Use this figure to communicate roughly what you want, not as the final print specification.
They are WCAG contrast ratios between this colour and white or black text. 4.5:1 is the AA threshold for body text, 3:1 covers large or bold text, and 7:1 is AAA. Below 3:1, text becomes hard to read for many people and should not be used.
Both start from hue and saturation. HSL's third value runs from black through the pure colour to white, which is why it suits generating tints and shades. HSV's runs from black to the pure colour, which is what most colour pickers show as a square gradient.
An alpha value in an rgba or eight-digit hex input is accepted but ignored — the conversions describe the opaque colour. Contrast in particular cannot be computed without knowing what is behind a translucent colour.