How it works
Every input format is parsed down to red, green, blue and alpha channels first, then re-formatted three ways. That shared internal representation is why any format can convert to any other, including translucent colors.
HEX vs RGB vs HSL
Hex and RGB describe a color as amounts of red, green and blue light — hex is just RGB written in base 16. HSL describes the same color by hue (a position on the color wheel), saturation (how vivid it is), and lightness (how close to black or white it is), which often maps more directly to how people think about adjusting a color: “a bit darker” is a lightness change, “more vivid” is a saturation change.
Alpha channels
An rgba(), hsla() or 8-digit hex value carries a fourth alpha channel for opacity, from fully transparent to fully opaque. Fully opaque colors are shown without an alpha channel, matching how they are normally written.