Out-of-gamut Colors
One of the artifacts that can occur with chroma subsampling is that out-of-gamut colors can occur upon chroma reconstruction. Suppose the image consisted of alternating 1-pixel red and black lines and the subsampling omitted the chroma for the black pixels. Chroma from the red pixels will be reconstructed onto the black pixels, causing the new pixels to have positive red and negative green and blue values. As displays cannot output negative light (negative light does not exist), these negative values will effectively be clipped and the resulting luma value will be too high. Similar artifacts arise in the less artificial example of gradation near a fairly sharp red/black boundary.
Filtering during subsampling can also cause colors to go out of gamut.
Read more about this topic: Chroma Subsampling
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