I got a definitive answer by an aviation plexiglass manufacturer. I don't know how to post a picture here of the UV graph of two different colors with and without extra UV protection. It looks like they all block UVB. It is UVA where they differ, from blocking only under 377nm, to blocking up to 400nm. So you won't get sunburned under any canopy, but you will get various amount of deeper skin damage from UVA based on your particular canopy.

Well, I guess I should have stated that this is data from one manufacturer..... Based on people getting tanned under their canopies, there maybe some that don't block UVB very well in addition to the UVA. Sorry about that.