To help prevent new wood from weathering, apply a premium penetrating oil finish with a high U.V. inhibitor rating such as “Penofin” by Performance Coatings). The finish should be reapplied periodically to maintain the desired color, depending upon the exposure to the sun and surface wear.
To restore mahogany to it's original color, you can power-washing with a commercial grade machine capable of 2500-3000 psi pressure. In some severe cases, a deck wood stripper can be used in conjunction with the power washing. This should remove most of the gray colored surface film and expose most of the original wood coloration. To maintain this appearance, follow with Penofin.
I'm always wary of advising people to power wash wood...some wood sin the mahogany family are hard, some are soft. If yours is a softer wood you can literally cut through it with a power washer, creating a bigger mess than you started with.
All you need to do is remove the outer layer of wood fibers that have weathered. Power washing is easiest, though if you do a poor job the results can be uneven. Removing via a mechanical method (sanding) would work as well. And it may actually be safer in terms of preventing additional damage...and give you a more even result.