Is it useful to paint outdoor conduit white to reduce the temperature?
Elevating the conduit 7/8” or more off the roof surface will be more effective than painting but yes white is the least heat absorbing color.  Why 7/8” elevation? That is what code requires or a temperature adder is used when derating the wire; with 7/8” elevation, the adder is not required. This is located in section 310.15(A)(3)(c), "Raceways and Cables Exposed to Sunlight on Rooftops".