Many of the exotic woods we work with have beautiful colors. How long will those color last in the pieces you buy? I have to say right from the start that red is an unstable color. You can see that in just about every town you drive through. There is always a store with a sign that has lost its red color. You can even find stop signs that are so bleached out they are no longer recognizable as red.
So where does that leave you with your beautiful new purpleheart slate frame, or padauk trestle stand? Most wood darkens with age. However, that trend does not lead all the way to black in most woods. Purpleheart starts out a grayish lavender color. By the time you get it, it is probably a lovely deep purple color. That color can last up to 10 years before it will settle into a dark brown. different boards will change color at different rates no matter how we treat them. Eventually though, your purpleheart and padauk are going to darken into brown.
There are a couple things that can be done to slow down that progression. First, when we make something large out of purpleheart, I give it a coat of spar varnish. This is one of the few finishes that contains a uv block. Then the piece gets another 3 or 4 coats of polyurethane. The more coats of finish the better. Now for what you can do. Purpleheart and padauk do not like sunlight. Try to keep them out of direct sunlight, or large amounts of reflected light. We could see the darkening of a padauk bowl that was kept in a white canvas tent for over a week. Enjoy the beautiful color while you have it.
The wood data-base has an article on color fastness that has a convenient list at the end showing the various woods and their colorfastness.
So generally love the color you have now, and try to protect it from direct sunlight.