Purple heart is a brilliant purple but will too soon turn bery dark and even brown in short time. any oils finish will darken the wood and with UV will turn very dark. If the bright purple is what you want to keep. Water based polyurathane will impart the least coloration. The water poly is a bad choice for figured woods since it wont bring out grain like other finishes. If you want to keep maple or oak bright, white and clear its a great choice. Extreamly durable. Look for a water poly with UV protection. The sunlight is the deal killer here.

I used to put a few coat of Armorall under the finish coat on padauk and purple heart to keep their color. Another trick is to stain or glaze the wood with its own color. you dont change anything, but it stays the same. For purple your going to need analyn dye or china pigments in a glaze. All of its lots of work and you'll need to do more reading on the topics if you choose that rout.