Sometimes enamel badges have a green rust or tarnish, and sometimes they are just very dull.
I have found that soaking them in vinegar for an hour, then a good rinse and dry, followed by a good polishing with Duraglit, will bring them up lovely and shiny, and gets rid of all the dullness and tarnish.
If the enamelling is stained or grubby, Cif bathroom cleaner rubbed on gently with a soft cloth and then a good rinse, will bring the enamelling up lovely and clean.
Sometimes sellers write the price on the back with marker pen, this can be removed with methylated sprits and cotton wool.
I hope you have found this useful, please vote YES for me at the bottom of the page.
All the best from Mandy.