In the end John you'll not stop purpleheart changing to a chocolate brown. You can delay it, but not stop it. The main causes of the colour change are:
1. Oxidation
2. UV damage.
Oxidation is simply air getting at the wood causing chemical change. UV light is sunlight getting at the wood.
The best way to protect the wood is a finish that seals as effectively as possible and has UV blockers as you mention. No finish is completely impervious to water ingress at a molecular (gaseous) level, but they all are effective to a greater or lesser degree. Broken down finishes fail to do the job, so any damage needs repair.
Water based varnishes are often formulated with good UV blockers, better than oil based in many cases. Water based varnishes dry clear but often have a cold, blue'ish sort of cast to them, and some have a sort of milkiness.
To defeat the cold look you can use a base coat of a dewaxed 'white' or 'super-blonde' shellac. This adds warmth to the wood. Importantly, it must be dewaxed otherwise a water based varnish won't stick to normal shellac. After a coat of shellac, go ahead and apply your varnish of choice-- just make sure it's one with good UV blockers.
I can't recommend a brand. I try to avoid using water based stuff because I haven't yet found a type that performs satisfactorily for me. I'm describing the final appearance that I find a wee bit ugly still, but others seem to like water based finishes well enough. Slainte.
