Colour & Tone
Colour and tone greatly contributes to how you feel about a photograph. We are all conditioned to connect a colour with an abundance of psychological and emotional meaning, and this can vary tremendously between cultural groups and individuals. For example blues and greens create a colder feel, see example below
Whereas if you look at this next photo which is simulaar to the one above I shot at a different time of day with warmer colours such as red it creates a very different feel.
The range of tonal values also effects a photograph. Large dark areas are are associated with strength and mood. Whereas images that are light suggest softness and space.
Try and use colour to help emphasize a subject. For example the image below would not have worked if the little girls dress were in another colour. Again simplicity makes more of a powerful photograph and that rule is applied to colour aswell.