Green branches preserved with glycerin often turn sort of a greenish-brown color, not quite as vivid as the fresh leaves but still pretty. The copper beech I used kept their burgundy colour but a lighter brown colour came through too which was really nice. As for how long they last, it’s possible that carefully-stored preserved leaves might last for years. However, if you’re preserving them for a wedding or special centerpiece, I’d recommend using leaves and branches that are as fresh as possible, preferably from the same season.
Good luck and I would love to hear any comments on how it went for you and any tips you might be able to add here. Thanks, Kathryn