All varnishes will add a little amber color. Technically you don't need any additional protection as the primer and exterior paint will do a good job. This is the same combo that is on your house.
If you still feel the need to add a coat or two of a clear you could try <a href="//www.painters-toys.com/varethane-exterior-water-based-finish-semigloss.html" target="_blank">Varathane's exterior water based spar urethane</a>. It doesn't seem to have the same amount of amber color. The semi-gloss would be a good choice.