Colour adds impact and interest, this could be complimentary or primary colours that work well together, shades and tones that are similar in colour, or a single highly saturated element within the image. The image on the left works particularly well because of the Primary colours of Red, Yellow and Blue. The image on the right works well because of the Complimentary colours of Green and Magenta (or purple).
These images have been made by excluding anything that does not fit with these colour schemes, leaving everything out of the frame that does not fit the colour scheme.
Colour can also be used in other ways:
A single colour element that stands out in an image.
Similar colours that make the image almost monochrome (single colour).