TBH this question must have come up 3 times in the last 6 weeks. All timbers darken and go more golden under exposure to UV light. It is a natural process and cannot be halted. Pine is particularly noted for its' propensity to yellow. All you can do is to use a clear, water-based UV-resistant coating and that will slow the process, not halt it. Avoid oils, as they are already golden in colour, and avoid oil-based "varnishes" as they not only start out straw to yellow coloured, but also the finish itself goes even more yellow over time