Image: Google Data Centers
Mood of an image is hard to measure... but with hue, saturation and luminance ("spectral shade", "colourfulness" and "brightness") you can get a good idea. I wrote a website that can analyse the colours present in any image you give it. You can try it out here.
So how about some simple examples? Lets try these images:
Blue, a wet car.
Green, a conifer frond.
Red, champagne bubbles.
These simple examples give a clear impression about how the plots work: in the middle is a circular histogram of the colours (spectral shades) in the image, and gives an idea of how much of each colour there is. Up the left is a histogram of image brightness (lightness of colour), and up the right is a histogram of colour saturation (vibrancy).
Together these three measures give an impression of the mood of an image: its key colours, how vibrant they are and how bright they are. The main colours of this image are easy to pick out:
Image: Komplex Graphix Wall Art
While my site lets you explore the colours that make an image appear vibrant and interesting it also reveals one of the tragedies of modern cinema; the teal/orange colour grading madness.
Screenshots from Transformers 2, Tron: Legacy, Hot Tub Time Machine, Iron Man and Wolfman.
So many blockbusters use orange and teal colour grading that a lot of modern films basically only have two colours: teal or orange. Bring back our other colours!
Image: Boston Big Picture
Paint.NET: screenshot management.