I sometimes find with GIMP auto levels that while the result is great for faded colours the contrast may be too high. In that case I increase the black output levels and possibly decrease the white output levels to reduce the harsh contrast. I have attached an example of auto input levels in GIMP working on one of my faded images. Obviously would need further tweaks but it gives the idea of how successful it can be for restoring faded colors with a simple click.
BTW, I have also worked with Darktables negadoctor which is a very handy tool if scanning or photographing negatives.
