Believe it or not, galvanized aluminum metal roofing is considered a 'cool roof' material with it's high reflectivity of about 60%. When we had our house re-roofed we went with a very similar galvanized aluminum propanel -
Cool Roof: Pro-Panel vs. Asphalt Shingle | EcoDaddyo.com.
I think there are about three things you could do to cool your shed:
1. Paint the roof white. That would certainly raise the solar reflectivity and that should help.
2. Vent the roof. I cleared out the eave vents and had a full ridge vent added to our roof. That allows the hot air to escape instead of just building up in the shed.
3. Add insulation beneath the roof. 1" or thicker foam sheet (maybe PolyIso foam with the foil layer facing up) beneath the roof should help block the heat radiating downward.
I think that insulation WITH venting between the foam and the roof would probably make the biggest difference.
FWIW,
Tim