I've made both black & white windshield covers for myself and others.
Both work well and I think it's largely a personal preference.
Visibility through the black was probably a little better but I agree the white is cooler.
If you are looking for a source for material Rochford Supply
I've found their prices attractive and they sometimes have good buys on 2nd & close-out fabrics. They will send samples of a few if you aren't sure which you are interested in.