The RGB grouping is for specifying colors by their red, green, and blue values. At some stage in this course I may ask you to select a specific color by its RGB values. In these cases, you would enter numbers in the corresponding numeric fields for R, G, and B. For example, the RGB values for 50% gray are R128-G128-B128 and would be entered as shown in the screen shot here.
The Lab grouping is for specifying colors by luminance. The L value specifies the luminance of a color. The A value specifies how red or green a color is. The B value specifies how blue or yellow a color is.
The CMYK grouping is for specifying colors as a percentage of cyan, magenta, yellow, and black, the colors used in commercial four-color process printing.
The # field is for specifying colors by their hexadecimal color codes. Hexadecimal color values are primarily used in Web design.