Using color models to describe color
We see only a small part of the electromagnetic spectrum. This small part is often called the visible spectrum. We see all light because light is defined as being that part of the electromagnetic spectrum that we can see. Color models attempt to describe the colors we see and work with. Each color model represents a different method for describing and classifying color. All color models use numeric values to represent the visible spectrum of color.
The range of colors that can be produced using a particular color model, such as RGB or CMYK, is a color space. Other color models are HSL, HSB, Lab, and XYZ.
A color model determines the relationship between values, and the color space defines the absolute meaning of those values as colors. Some color models have a fixed color space (such as Lab and XYZ) because they relate directly to the way humans perceive color. These models are described as being device independent. Other color models (RGB, HSL, HSB, CMYK, and so forth) can have many different color spaces. Because these models vary with each associated color space or device, they are described as being device dependent.
For example, the RGB color model has many RGB color spaces, such as ColorMatch, Adobe RGB, sRGB, and ProPhoto RGB. You can take the same RGB values (R=220, B=230, and G=5) and assign profiles that describe different RGB color spaces. The color will look different in each color space, but the numeric values and model will still be the same.
Photoshop uses color modes (similar to a color model) that let you work with an image in a specific color space. Photoshop keeps track of an image's color space and will indicate in the title bar if the working space and the document's color space don't match.
Photoshop accommodates the color models that are most suitable for photographic images or graphics. Color models used in Photoshop are CMYK, HSB, RGB, and Lab.