Quick answer: Unreal FBX import putting vertex colors in the wrong channel? FBX exporters may swap R/B; check import settings or remap in material.
Painted vertex colors in Blender: red trim, blue accents. Imported: blue trim, red accents.
Check import settings
FBX importer > Vertex Color Import Option = Replace. Some FBX exports use 'Override' which swaps; explicit 'Replace' preserves the source order.
Remap in material
Multiply vertex color by a swap matrix. Cleanest if the swap is reliable.
Bake vertex colors before export
Blender's vertex color export is configurable. Setting the right export channel order saves the import-time fight.
“Vertex colors are channel-ordered. Channel order is a contract; contracts can disagree.”
Establish a vertex color spec for your project. Channel meanings; export settings; import expectations. The doc prevents the bug.