Pick a color and copy it as HEX, RGB or HSL.
Use the color picker, then copy the format you need for CSS, design tools or documents.
They describe the same color. HEX writes the red, green and blue values in base 16, for example #2F5BFF, while RGB writes them as numbers from 0 to 255.
HSL (hue, saturation, lightness) is easier to adjust by hand, for example to make a color lighter or less intense.