Glossy and semi-gloss varnish recreate the look of the wet paint, as well as make the colors look deeper and a little juicier. However, varnish does not add coloration to the paint. If the painting for some reason has faded, varnishing it will not bring it back to the original look.
If you are dealing with an old painting, I would not apply any varnish unless you are confident that 1) The surface is clean from any dust or residue; 2) The surface has no previous varnish application.
If you are trying to restore it, I recommend talking with an expert, or you may end up making the damage worse.