Why does my cutout have a white or coloured halo around the edge?
Short answer
The halo is colour from the old background bleeding into semi-transparent edge pixels — most visible when a cutout made against white is placed on a dark background. Fixes: shoot against a contrasting plain wall, check the result on a dark backdrop before exporting, and re-run with a sharper original.
Anti-aliased edges are a blend of subject and background colour. A good matte reduces this, but a light background always leaves some light fringe, and a dark one a dark fringe. That is why product photographers shoot on a neutral grey when the final backdrop is unknown.
The halo fix guide covers the details, including defringe steps in common editors. In the bgclear editor, adding a thin outline or a shadow often hides a faint fringe entirely.