The colors in digital images are necessarily constrained to quantized values by the digitisation process. This quantization of the CIE color spaces produces fundamental geometric patterns. Pixels from real digital images must lie within these patterns, resulting in geometric artefacts. These artefacts are intensified by errors in the sampling such as misalignment of the color planes. The shape of the artefacts may indicate the processing history of the image.
Keith Godfrey, "Digital Color Quantization Patterns in CIE Spaces and Artifacts from Misaligned Color Planes" in Proc. IS&T 6th Color and Imaging Conf., 1998, pp 209 - 213, https://doi.org/10.2352/CIC.1998.6.1.art00045