Advances in colour image sensors and computer technology have opened up new opportunities for colour-based image analysis by allowing it to use multispectral images - images that have tens or even hundreds of spectral colour channels. As multispectral images usually occupy large amounts of memory, some suitable way is needed to compress such images and to represent them efficiently. Image compression has been one of the mainstream research topics for a long time; however, the research usually focuses on compressing images that are intended to be finally seen by humans. While many traditional methods can also be reused for multispectral images, some features of the multispectral images can be addressed differently than in traditional images. This paper describes a way of how to represent the multispectral colour data as a linear combination of colours contained in the clusters of colours that most frequently occur in an image.