Sail colours: if white isn't right, why is it so popular?

We ordered two new sails in Marmaris, Turkey. They had to be brown ( Tanned ) to match the other two sails. They ware 30 % more expensive than white. Still have Cataract. Always protected myself. Hat, shirt, staying in the shade, but did not like the fly-eyes sunglasses. Result cataract. Now I ware strong sunglasses but walking on white decks, especially catamarans, I have problems seeing the different levels of the deck.*
Our deck is teak, better for the eyes but hot on the feet in summer.*
I learned from Marlow, the rope maker that white is bad for UV. The sales teacher told us that white reflects the UV and so guides the UV into the rope ( Sail cloth ) Dark surface absorbs but no transmission of UV. Just the outer layer suffers.*
 
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