Oil based varnish gets darker with age and that's a fact. The sap in wood gets darker with oxygenation and UV exposure.

My comment is about appearance and not about chemistry. If you have light wood covered with oil based varnish, it will get darker and darker. The darkening of varnish will far surpass and darkening of the wood itself. Most commercial wood musical instrument makers use polyester for that reason. A few old school builders use oil based varnish and just don't care about preserving the lightness of the wood. Your example is correct. If you tape over a piece of wood no UV or O2 is hitting it. Varnish gets darker in the can with just O2. Plus kayaks bake in the sun all day long which is the extreme conditions suitable for the darkening of varnish and epoxy. Quite frankly, I don't know if there is a way to prevent light wood from darkening on a kayak other than a polyester finish and there it might just slow it down quite a bit. A less sappy wood would be slower in turning color than perhaps pine. Maybe white maple? Perhaps this is why musical instrument makers like holly?