I think grey and brown are the best color to prevent UV rays. The two colors are also the most common color that are used in sunglasses.So if you want to get sunglasses, you can try the two kinds of color.
It is depend on what you ues your sunglasses for. Usually the grey is the best color to prevent UV rays to the maximum. Gray is the best choice if you want to see the colors of things in the most natural and accurate way. All gray sunglasses do is remove some of the light of all wavelengths (colors) in equal amounts.