Today’s Question: I recently read a reference to HSL as a color space. I had never heard of this. How does the HSL color space relate to RGB, and is there any need for me to consider working in HSL?
Tim’s Quick Answer: HSL (or HSB) is actually a color model, rather than a color space. A color model is a method of describing colors, while a color space is a range of colors available for use in an image. What that means is that HSL simply provides a different way to describe color compared to the RGB color model.
More Detail: When using the HSL color model, colors are described based on their hue, saturation, and luminance (or brightness). That typically involves a hue value expressed as degrees around the color wheel, and values for saturation and luminance (or brightness) expressed as a percentage. In the RGB color model the same colors are described based on the amount of red, green, and blue light are combined to create a color. Those RGB values would range from zero to 255 for 8-bit per channel images, and from zero to 65,535 for 16-bit per channel images.
In other words, when working in the RGB model, you could also say that you are working in the HSL model. Both simply provide different ways of describing the same colors. In software such as Photoshop, you aren’t able to switch to the HSL color mode. Rather, you would typically work in the RGB color mode. However, in the Color Picker dialog (and elsewhere) you can see HSL values (referred to as HSB values in the context of Photoshop) in addition to RGB values. There are also other options included, based on other color models, including Lab and CMYK.
When working in software such as Photoshop, you can choose a color space for your photos. So color spaces such as ProPhoto RGB, Adobe RGB, and sRGB describe a specific range of available colors, while a color model determines how those colors are communicated.