It depends - on how much light the wood sees and how much the finish yellows. If you had Vermilion along side that Purpleheart it would probably brown out at about the same rate.
A board of Vermilion or Purpleheart set out in the sun with no finish will brown out in one day. Keep your instrument in the case when not in use and it will retain color longer. The newer finish doesn't yellow like the old one, sometimes the added layer of yellow makes the brown look even more extreme.
If you sand the finish off and sand a few cells off the wood, it will brighten up again, it's only the outside layer than changes dramatically.
