I don't know about the degree of UV protection offered by tinting. In terms
of how dark can one tint, I have added black color to make the epoxy as dark
as ebony. By the way, I have had a test sample of epoxy and fiberglass
scarfed plywood in the bed of my truck for four years with no apparent loss
in strength of the joint. The (clear) epoxy has yellowed a bit but the joint
seems to be very strong.
Dave