Most digital cameras use a Bayer colour filter array to capture colour images. Each pixel's sensor captures only one colour of filtered light, and the colour filters are arranged in a periodic pattern over the sensor. As a post-processing step (in the camera firmware, or in a raw file converter, for example) the missing (filtered) colour components are interpolated from each pixel's neighbours.

An investigator can try to work out what interpolation method was used via a statistical analysis of the image's pixels. This can give an indication of what camera make/model or post-processing software was used to produce a untampered test image.

Areas of the image that are inconsistent with an expected interpolation pattern may have been tampered.

There is more information in my slides on colour filter array interpolation detection.