The value of a color is simply the lightness or darkness (in black-and-white) of that color. If you were to take a black-and-white photograph of that color, it would be one of the values on the value scale. Red is always a dark value (7-9), and yellow (2-4) is always a light value. Yellow ochre is more of a mid value (4-6). If you painted a panel with all your favorite colors, and then took a black-and-white photograph of them, you would see the values of your favorite colors.