Color bleeding is color smearing past the sharp edge it is supposed to sit inside - color leaking outside its lines, shown as a magenta or cyan ghost along a saturated border. It happens because codecs compress color harder than brightness on purpose: the eye is far more sensitive to luminance than to chrominance, so chroma subsampling (most commonly the 4:2:0 scheme) stores a full-resolution brightness image but only a quarter-resolution color image. Usually invisible, but on a saturated, high-contrast color edge - red text on grey, a bright logo - the low-resolution color cannot keep up with the sharp brightness edge and smears. Yuen and Wu pinned the mechanism to the quantizer zeroing the higher-frequency color coefficients, leaving only a coarse low-frequency version of the color. Saturated reds and colored text on screen content and presentations are where it shows. The catch for measurement is that luma-focused metrics under-weight chroma, so PSNR, SSIM, and VMAF catch it only weakly.
