More on Value Scale & Tonal Range

Value -- the range of light and dark within either neutrals or colors. Black is at one extreme, white at the other.

In painting, value changes can be achieved by adding either black or white to the chosen color.
However, this also reduces the saturation and adds to the dilution of the color.
Believe it or not, value is more important than color to the design and success of a painting.
Here’s why:
• Value is used to create a focal point within a painting or drawing.
•The human eye is immediately drawn to a light element against a dark element. This creates, the focal point of interest.
•To create the illusion of depth, gradations of value are also used.
•Areas of light and dark give a three-dimensional illusion of form to subject matter.


Value is independent of its hue.  

Tone -- a color mixture that is not pure color hue (such as the highest chroma for a given color), and not black and not white. This mixture is an intermediate (can also be blend of hues). In other words, if you were to take a pure color and add grey, you would have Tone.

Continuous Tone Photos:

Composition - why tonal values and contrast are important

http://makingamark.blogspot.com/2008/02/composition-why-tonal-values-and.html

References:
http://www.artistsnetwork.com/subject/other-subject/understanding-value-and-tone-for-better-painting
http://photography.tutsplus.com/articles/light-photography-exposure-and-tonal-range-considerations--photo-5685