Any mild acid will clean up the brass (from vinegar to ketchup) - but the vinegar will be easier to rinse off afterwards !
Enamel is just glass - ground, powdered silica (sand) mixed with various oxides to add colour and then bonded by fusion to a heat-softened metal surface in a kiln, so I would think just warm soapy water should clean this part up. Use a toothbrush to remove any dirt in crevises and be very gentle.
You could always take the lamp to your local chandler and ask for their advice if you're not sure.