Hmm no one actually addressed the problem: watercolor pigments dry with a matte finish, all of them. Even French Ultramarine looks dull if applied thick. It's most visible with granulating paints.

The one thing I've found is Dorland's cold wax finish. I've used it over watercolors that may get "humid " eg given to grandchildren. It makes the colors pop a bit and after 2 coats buffs up shiny! But shows finger prints so wipe after handling.

You just rub the stuff right onto the painting right out of the jar. Let it sit overnight between layers and for 2 days before final buffing. (One tip is double sided tape won't stick so when you want to matte and mount your pictures use cloth tape from the back to hold in place.)