When a Static Mesh is reimported with a different vertex count than the number of vertex colors on the instance, you get errors like this in map checks and during cooking:
StaticMeshActor_73 (LOD 0) has hand-painted vertex colors that no longer match the original StaticMesh
Mismatched instanced vertex colors against the mesh's vertices makes the vertex colors simply nonexistent on console and look wrong on PC. To remedy that, the Fix button in the Paint Mode Dialog lets you fix up vertex colors if they do not match up with the Static Mesh.
This feature will only be available if a mesh needs fixup; otherwise, it will be disabled. The tool works well in most cases especially when you are only making small tweaks. The more drastic the change to the mesh, the more unlikely the colors will match. The feature is designed to always match some color even if the changes are drastic. We elected not to make this tool automatic because that would prevent people from easily opting out of the fixup if they did not like the results. Additionally, adding the feature to mesh paint means you can easily touchup after the fix is applied.