time and wear will darken leather. Unless the color difference is tremendous you may find they age together.
Neatsfoot oil preserves and darkens it a bit as well.
Re-dying after leather is sealed with a sealer is a bit tricky... the sealer keeps the leather from penetrating (which is what it is supposed to do from the inside out, but it works both ways).
Neatsfoot oil will penetrate sealer, especially from the back side (which is rougher). Even applied from the back it will eventually affect the front. However, a liberal coat applied on the front left all night will penetrate the front side and darken it a bit.
If you do try to use dye to darken it,firts rub with mineral spirits to remove the sealer. Mix the dye with a bit of mineral spirits 50%-50%. That will thin it so that it does not overdye it. If the leather is dry after all this, neatsfoot oil will soften it again. You will have to re-seal it again afterword. Sealer can be applied after softening a bit with neatsfoot oil, as long as the oil is not overdone before sealing.