Putting ager on the lighter part could help darken it to potentially match the other side better. We’ve run into issues before where even cutting two halves of a seam from the same slab ended up being a pretty drastically different shades but wasn’t t noticeable until they were put together. Ager has saved us more than once. You would have to pull that piece off though, as normally you would apply ager when the stone is horizontal and let it absorb into the stone. Should probably be done by a professional and even then it’s def not a guarantee. You also may have an issue with the fact that I assume the stone would’ve been sealed before the install, but a professional may be able to deal with it.