Any oils that have absorbed into the wood will darken with age. Additionally, varnishes will have different pigments and finishes from brand to brand, and even batches or age of materials. You will have to experiment in areas to see what you like and how it fits your idea of what you want. Make sure to use plenty of ventilation and get plenty of fresh air after cleaning up all the dust that you can. Let us know what you end up using and please post some picture!
s/v Blues Harp