The Value of an image is measured by the quantity of light that it reflects, hence its apparent lightness or darkness. The degree of lightness is measured on a scale of grays running from black to white, with darker tones lower in value than lighter ones.
Stated another way, value is the organization and magnitude of light and dark in your photo, irrespective of color values.
In most photos, value is roughly balanced. The number of strong white areas and strong dark areas are about equal. We do this by instinct. However, Value can be tipped out of balance in order to provide meaning and visual interest.
Intensity is measured by the quality of light, i.e., the brightness or dullness of an image. An object's Intensity is relative to its surroundings, while Value is not.
High intensity colors include: kiwi, lime, tangerine, lemon
Low intensity colors include: adobe, butterscotch, tortoise, forest, nightfall
As objects shrink in size, their colors become less distinct. Dark colors, such as blue, converge on black while bright desaturated colors, such as yellow, become whiter.
Contrast is the difference between the intensity or value of two colors in a color scheme.
The reflectance of all objects in an outdoor scene averages out to a neutral gray reflectance value of 18%. Accordingly, exposure meters inside a camera are set for a 18% gray Value. Meters do not care about color, just the value of light within the measured area of the scene. The camera will then calculate the proper exposure settings (shutter speed and f-stop).
If you point the camera and take a reading, the camera will assume that the measured area is a medium brightness level (i.e., the 18% reflectance neutral gray). Aim the camera at a bright area within a scene and the camera will read it as a mid-tone. Aim at a dark shadow and the camera will read that shadow as a mid-tone.
Aim the camera metering system at the bright white of a wedding dress and it will be rendered as duller white with all other values underexposed. Aim at the black tuxedo, and the it will appear less black, with the wedding dress overexposed.