Hue

Hue also has value. “When contrasting hues are made similar in value, the spatial effects are flattened out. The pair of images above demonstrates this. In the color image of the fashion model the coat draws our attention through contrast of hue although the skin tones blend with the background (remember the object of the image is to sell the coat, not the model).”
Ref. http://char.txa.cornell.edu/language/element/color/color.htm  

The soft blending with the background has flattened the dimension between the background and object. The projection of the figure along with the coat required greater contrasting color value. In the examples of the color strips (Fig. 9) the hues of brown and green are so close because of the value/ intensity that they completely get flattened.

Therefore, if values are too close, shapes will lose the dimension and would get flattened. In result objects would lose their depth of field. If values contrast, shapes will appear to separate in space and some will stand out from the others.

Subtractive and Additive Colors

Commonly spectrums of colors are known as ‘hues’ such as- red, orange, yellow, blue, green violet. Mixing primary colors create various hues. When pigment primaries are all mixed together, the theoretical result is black; therefore, pigment mixture is referred to as Subtractive Color. When we mix colors using pigments, or through the printing process, we are using the subtractive color method. Subtractive color mixing means that one begins with white and ends with black; by adding color, the result gets darker and tends to black.
On the otherhand if we work with light-colors on computer is known as Additive Color. The application of Additive Color mixing is commonly on computer.  Additive color mixing begins with black and ends with white; as more color is added, the result is lighter and tends to white.
(Read more: http://www.worqx.com/color/color_systems.htm ; May 29, 2012)