1. A method of representing a group of images using at least one processor to process signals corresponding to the images, the method comprising: obtaining, using a processor, an image descriptor which is an image dominant colour representation for each image in the group of images, the image descriptors expressing the images in terms of one or more dominant colour values, and where each of the image descriptors has one or more components including at least one dominant colour value, and optionally for each dominant colour value, a variance indicating colour distribution variance of the image with respect to the dominant colour value, a weight indicating the influence of the dominant colour in the image, the number of dominant colours in the image, and spatial homogeneity of pixels corresponding to the dominant colors in the image; deriving, using a processor, a group descriptor which is a group dominant colour representation expressing the group of images in terms of one or more of said dominant colour values by aggregating a plurality of said image descriptors by combining, in a first stage, components for which the distance between the respective image dominant colours satisfies a predetermined condition, and combining other components in a second stage; and storing the group descriptor as a descriptor for the group of images, wherein said first stage comprises a first iteration of merging dominant colour values, and said second stage comprises at least one further iteration of merging dominant colour values, thereby successively reducing the number of dominant colour values.