Chroma is the color information of a video signal, carried by the two chrominance channels Cb and Cr in the YCbCr representation, as distinct from the brightness channel luma. Together with luma it reconstructs the full-color picture. Because the human eye sees brightness detail far more sharply than color detail, chroma is usually stored at lower spatial resolution than luma, a saving exploited by chroma subsampling: the common 4:2:0 scheme halves chroma resolution both horizontally and vertically, cutting color information by about seventy-five percent. The catch is that this saving is usually invisible on faces and landscapes but shows up as color bleeding on saturated red text or sharp color edges, exactly the content where reduced chroma resolution becomes a visible quality-loss point. Many full-reference metrics also weight chroma less than luma or look at it only weakly, so color artifacts can slip past a brightness-focused score. Chroma is the counterpart to luma, and the asymmetry between them drives subsampling decisions throughout the pipeline.
